Abstract:
Systems and methods are providing for searching for documents within topically-defined clusters. A search space is defined, starting with one or more source documents, by examining references from one documents to another and following the networks of references to some level of indirection. Depending on the embodiment, references may be followed from a document containing a reference to a referred-to document, or from a referred-to document to a document containing a reference, or both. Once a search space has been defined, a query is executed, and documents within the search space that satisfy the query parameters are identified. 
     In certain embodiments of the invention, the documents primarily relate to legal materials, and one or more source documents are associated with one or more topics within a topic directory. In such embodiments, a search query may be limited to one or more selected topics by executing the search query within a search space defined using the associated document or documents as the source.

Description:
COPYRIGHT NOTICE 
       [0001]    A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyrights whatsoever. 
       BACKGROUND 
       [0002]    The invention disclosed herein relates to computerized searching for electronic data within collections of stored data, such as, e.g., documents. 
         [0003]    Electronically stored data, or information, is now available in immense quantities, and the rate of growth is accelerating. Such data may be stored as logical units of various kinds, such as, e.g., documents, files, records, etc. 
         [0004]    A simple example is the World Wide Web (the “Web”), a global information space comprising hyperlinked documents that are requested and distributed via the Internet. From its origin in 1990, the Web has grown such that a study conducted in January 2005 concluded that it comprised at least 11.5 billion indexable Web pages alone. 
         [0005]    Specialized databases continue to grow as well. For example, electronic legal databases store, e.g., statutes, regulations, judicial opinions, and secondary sources and are constantly updated an expanded. Examples of such legal databases are described in commonly-owned U.S. patent application Ser. No. 10/045,586, filed on Jan. 11, 2002, and titled “DYNAMIC LEGAL DATABASE PROVIDING CURRENT AND HISTORICAL VERSIONS OF BODIES OF LAW,” and commonly-owned U.S. patent application Ser. No. 10/603,207, filed on Jun. 25, 2003, and titled “ELECTRONIC MANAGEMENT AND DISTRIBUTION OF LEGAL INFORMATION,” both of which are hereby incorporated by reference herein in their entirety. 
         [0006]    Data stores of these sizes would not be useful without tools for searching for and retrieving desired information. Various types of search tools (commonly referred to as “search engines”) are well-known. Typically, a search engine accepts a query from a user and then tries to identify all data that in some way corresponds to the query. A list of some or all matching logical units that contain data responsive to the query is provided to the user, who may then be able to retrieve some or all of the logical units. 
         [0007]    The utility of a search tool, however, often depends on how well the user can formulate a query. For example, one query related to a topic may return little or no useful information, while a slightly different query may return hundreds, or even thousands, of hits, which may be far too many to examine. Users may waste considerable time in trial and error before stumbling upon a query that leads to a manageable number of relevant hits. In practice, a user may settle for a query known to be overinclusive and then waste additional time mining the results. Against this background, a user may wish to reduce the set of searchable data, and one approach is to limit the search to a subset comprising documents that are related to one another by topic. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    The invention provides for limiting a search of a data set, e.g., a collection of logical units such as documents, to a subset thereof, which may be referred to as a “search space.” Membership in the subset may be determined by one or more relationships between and/or among the among the data, such as, e.g., relationship to one or more common topics. Embodiments of the invention provide for defining a topically-related data subset in which a search may be conducted for data responsive to a query. 
         [0009]    According to embodiments of the invention, data in the data set contains references to other data in the data set, originating data is selected within the data set, and references to and/or from the originating data are used to define a subset of the data set within which a search may be conducted. In an embodiment of the invention, the subset comprises the originating data and data referred to by and/or referring to the originating data. In embodiments of the invention, the subset may comprise further data that is identifiable by iteratively following references to and/or from the originating data. For example, considering the data to be nodes in a graph, and further considering references to be edges in that graph, the originating data is represented by one or more originating nodes, the first iteration adds nodes at a distance of one from an originating node, the second adds nodes at a distance of two, and so on. 
         [0010]    In an embodiment of the invention, a method is provided for defining a subset of a searchable data set, the method comprising defining the subset to include originating data and, for at least one iteration, further defining the subset to include the union of the data currently defined to be in the subset and all data to which any data currently defined to be in the subset contains at least one reference. In an embodiment of the invention, a method is provided for identifying data within a searchable data set, the method comprising defining a subset of a searchable data set, as before, and identifying data within the subset that satisfies one or more criteria of a specified search query. In a further embodiment of the invention, the originating data is associated with one or more topics, and the originating data is specified by selecting one or more topical areas for searching. 
         [0011]    In an embodiment of the invention, a method is provided for defining a subset of a searchable data set, the method comprising defining the subset to include originating data and, for at least one iteration, further defining the subset to include the union of the data currently defined to be in the subset and all data known to contain a reference to any data currently defined to be in the subset. In an embodiment of the invention, a method is provided for identifying data within a searchable data set, the method comprising defining a subset of a searchable data set, as before, and identifying data within the subset that satisfies one or more criteria of a specified search query. In a further embodiment of the invention, the originating data is associated with one or more topics, and the originating data is specified by selecting one or more topical areas for searching. 
         [0012]    Embodiments of the invention involve the use of references from one document to another to identify sets of documents likely to relate to one or more common topics and to limit a search of a larger data set to such a topical search space. Embodiments of the invention are described in more detail herein with respect to data in units arranged as documents. But as indicated above, the invention and the various embodiments thereof described herein apply equally to many other arrangements of data, including, for example, data arranged into logical units such as, e.g., files, records, etc. Also, embodiments of the invention are discussed in connection with databases of legal materials, but the invention and the various embodiments thereof disclosed herein apply generally to any sort of collection of data that may contain references to other data within the collection. 
         [0013]    In an embodiment of the invention, a computerized system is provided for identifying one or more electronic documents within a collection of electronic documents, the system comprising at least one interface; and at least one processor coupled to the at least one interface and programmed to (1) accept a search query through one of the interfaces, (2) obtain a definition of a subset of a collection of electronic documents that comprises a plurality of electronic documents, (3) execute the search query within the subset, thereby obtaining any results responsive to the search query, and (4) provide any result or results through one of the interfaces. In such an embodiment, obtaining a definition of a subset comprises defining a subset to comprise (1) at least one source document within the collection, each of the source documents comprising at least one reference that identifies an additional document within the collection of documents, distinct from the source document, and (2) further additional documents identifiable by, for some number of iterations, for each additional document added to the subset in the immediately preceding iteration: (a) retrieving the additional document, (b) finding in the retrieved document one or more references, each of the one or more references identifying an additional document, and (c) adding each of the found references, not in the definition of the subset, to the definition of the subset. In a further embodiment of the invention, the subset comprises every electronic document within the collection that any electronic document within the subset comprises a reference to. 
         [0014]    In an embodiment of the invention, a computerized system is provided for identifying one or more electronic documents within a collection of electronic documents, the system comprising at least one interface; and at least one processor coupled to the at least one interface and programmed to (1) accept a search query through one of the interfaces, (2) obtain a definition of a subset of a collection of electronic documents that comprises a plurality of electronic documents, (3) execute the search query within the subset, thereby obtaining any results responsive to the search query, and (4) provide at least one of the results through one of the interfaces. In such an embodiment, obtaining a definition of a subset comprises defining a subset to comprise (1) at least one source document within the collection, (2) additional citing documents identifiable by, for some number of iterations, for each document added to the subset in the immediately preceding iteration: (a) finding one or more additional citing documents in the collection, each comprising at least one reference to the document, and (b) adding each additional citing document, not already in the subset, to the subset. In a further embodiment of the invention, the subset comprises every electronic document within the collection that is known to comprise a reference to any electronic document within the subset. 
         [0015]    In an embodiment of the invention, a method is provided for identifying one or more documents within a collection of documents, the method comprising defining a subset of a collection of documents, the collection of documents comprising a plurality of documents, and the subset comprising (1) at least one source document within the collection of documents, each source document comprising at least one reference that identifies an additional document within the collection of documents, distinct from the source document, (2) additional documents identifiable by, for some number of iterations, for each document added to the search space in the immediately preceding iteration: (a) retrieving the document, (b) finding in the retrieved document one or more references, each of the one or more references identifying an additional document, and (c) adding each of the found references, not already in the definition of the search space, to the definition of the search space; accepting a search query comprising one or more criteria; and identifying one or more documents within the subset that satisfy the one or more criteria comprised by the search query. In a further embodiment of the invention, the subset comprises every electronic document within the collection that any electronic document within the subset comprises a reference to. 
         [0016]    In an embodiment of the invention, a method is provided for identifying one or more documents within a collection of documents, the method comprising defining a subset within a collection of documents, the collection of documents comprising a plurality of documents, and the subset comprising (1) at least one source document within the collection of documents, (2) additional citing documents identifiable by, for some number of iterations, for each document added to the subset in the immediately preceding iteration: (a) finding one or more additional citing documents each comprising at least one reference to the document, and (b) adding each additional citing document, not already in the subset, to the subset; accepting a search query comprising one or more criteria; and identifying one or more documents within the subset that satisfy the one or more criteria comprised by the search query. In a further embodiment of the invention, the subset comprises every electronic document within the collection that is known to comprise a reference to any electronic document within the subset. 
         [0017]    In an embodiment of the invention, a method is provided for defining a topical subset of a collection of documents, the method comprising defining a subset of a collection of documents to comprise at least one source document within the collection of documents, each source document comprising at least one reference that identifies an additional document within the collection of documents, distinct from the source document, wherein this defining comprises a first iteration, and defining the subset to comprise additional documents identifiable by, for some number of iterations, for each document added to the search space in the immediately preceding iteration: (a) retrieving the document, (b) finding in the retrieved document one or more references, each of the one or more references identifying an additional document, and (c) adding each of the found references, not already in the definition of the search space, to the definition of the search space. In a further embodiment of the invention, the subset comprises every document within the collection that any document within the subset comprises a reference to. 
         [0018]    In an embodiment of the invention, a method is provided for defining a topical subset of a collection of documents, the method comprising defining a subset of a collection of documents to comprise at least one source document within the collection of documents, such defining constituting a first iteration; defining the subset to comprise at least one additional document identifiable by, for some number of iterations, for each document added to the subset in the immediately preceding iteration: (a) finding one or more additional citing documents within the collection of documents, each additional citing document comprising one or more references to the document, and (b) adding each of the additional citing documents to the subset. In a further embodiment of the invention, the subset comprises every document within the collection that is known to comprise a reference to any document within the subset. 
         [0019]    In some embodiments of the invention, at least one of the source documents pertains to at least one area of law. In some further embodiments of the invention, the provided method comprises selecting at least one topic from a directory of areas of law, wherein each source document pertains to at least one of the selected topics. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The invention is illustrated in the figures of the accompanying drawings, which are meant to be exemplary and not limiting, and in which like references are intended to refer to like or corresponding things. 
           [0021]      FIG. 1  is an excerpt from a textual document that contains citations according to the prior art. 
           [0022]      FIGS. 1   a  and  1   b  are additional views of citations that appear in  FIG. 1 . 
           [0023]      FIG. 2  is an excerpt from an HTML document that contains hyperlinks according to the prior art. 
           [0024]      FIG. 3  depicts possible relationships between documents according to the prior art. 
           [0025]      FIG. 4  depicts the relationships of  FIG. 3  as a directed graph. 
           [0026]      FIG. 5  depicts creating an N-level search space according to an embodiment of the invention. 
           [0027]      FIG. 6  depicts creating an exhaustive search space according to an embodiment of the invention. 
           [0028]      FIG. 7  depicts a topical search according to an embodiment of the invention. 
           [0029]      FIG. 8  depicts a user interface display from which a user may select a topic. 
           [0030]      FIG. 9  depicts a user interface display from which a user may enter a search query. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0031]    Embodiments of the invention relate to searching for electronic documents according to one or more criteria. Many aspects of the invention and of particular embodiments of the invention are discussed herein in connection with legal documents, including, e.g., judicial opinions, statutes, and secondary sources such as legal treatises and case annotations. The discussion of legal documents is provided purely as an example, however, and does not limit the scope of the invention. It will be recognized by those skilled in the relevant arts that the subject matter of the invention is applicable to documents of widely varied types. 
         [0032]    A document may be considered a container of data that may be indexed and/or retrieved as a unit in a data management system. Although it is common for a document to be a single computer file, this need not be the case. It is well known for a document to comprise several files: for example, a page on the World Wide Web may be considered a single document, but it can comprise resources stored in several files. Conversely, it is also well known for a file to comprise several documents: for example, in a database management system, multiple documents may be stored as records within a single file. 
         [0033]    An electronic document may comprise digital data representing one or more types of information. (As a shorthand, it may be said herein that the document contains or comprises the information.) For instance, documents commonly comprise human-readable text, but may also comprise recorded sound, still and/or moving pictures, and/or other types of information in addition to or instead of text. A document may also comprise data intended for automatic processing, such as, e.g., formatting codes and/or markup in a language such as, e.g., XML or HTML. 
         [0034]    A document may refer to one or more other documents. Such a reference may take the form of text intended to specify the referred-to document with sufficient precision that a human reader is able to identify the referred-to document unambiguously. Such a reference may sometimes be called a “citation,” a document containing a citation may be referred to as a “citing document,” and the document specified by the citation may be referred to as a “cited document.” 
         [0035]    Standard forms exist for human-readable citations. Well-known standards are published by the Modern Language Association, the American Psychological Association, and the University of Chicago Press. Legal documents often adhere to the conventions described in The Bluebook: A Uniform System of Citation (Columbia Law Review Ass&#39;n et al. eds., 17th ed. 2000) or Association of Legal Writing Directors, ALWD Citation Manual: a Professional System of Citation (Darby Dickerson ed., 2d ed. 2002). 
         [0036]      FIG. 1  contains a fragment  100  of a document containing legal citations  105 ,  106  in a standard style. A case citation  105  refers to an opinion or other document issued by a court.  FIG. 1  a depicts the parts of the case citation  105 . As depicted, the case citation  105  comprises a title  110 , which often comprises the names of one or more parties to the action, an abbreviation  111  identifying the series of volumes (often called a “reporter”) in which the opinion is published, the number  112  of the specific volume in which the opinion appears, and the first page  113  of the opinion within the volume. A court opinion is often cited as authority for a particular proposition of law, and a case citation  105  may therefore also comprise a “jump cite”  114  that indicates the page or pages on which the opinion provides that authority. A case citation  105  commonly also comprises the date  115  on which the court rendered the opinion. 
         [0037]    Other information may appear in a case citation  105  in addition to and/or instead of the information depicted in  FIG. 1   a . For example, when not clear from the identity of the reporter, the case citation  150  may indicate the court that rendered the cited opinion. For another example, if an opinion is published in more than one reporter, the case citation  150  may indicate the multiple publications by including “parallel citations” (not pictured). 
         [0038]    The case citation  105  depicted in  FIG. 1   a  is sometimes referred to as a “long form” citation. Abbreviated forms of citation (not pictured) exist and sometimes follow the long form citation when the same document is cited more than once. 
         [0039]    A statutory citation  106  identifies one or more sections of law. As depicted in  FIG. 1   b , the statutory citation  106  identifies a code of laws  120  into which the particular section has been codified, the title  121  within the code that contains the cited material, and one or more sections or parts of sections  122  within that title. Other information may appear in a statutory citation  106  in addition to and/or instead of the information depicted in  FIG. 1   b , such as, e.g., the year in which the code was published, the name of the Act containing the section, etc. 
         [0040]    Standard forms exist for citing many other types of documents. Style manuals commonly prescribe long forms and abbreviated forms for citing nearly every type of document that a legal document might refer to. 
         [0041]    The citation forms depicted in  FIGS. 1 ,  1   a , and  1   b  were developed for use by human readers. Although computers can be and have been programmed to recognize these forms in electronic documents, other kinds of references are often used when intended for electronic processing.  FIG. 2  depicts the use of one type of machine-readable reference, called a “hyperlink,” that may be used in conjunction with, e.g., the World Wide Web. 
         [0042]    The document  130  has been marked up using the HyperText Markup Language (“HTML”). Plain text  134  is marked up with tags  135 ,  137 - 38 ,  140 , which delimit one or more elements and may indicate, e.g., the structure of the document  130 , the meaning or significance of one or more portions of the document  130 , and/or the appearance and/or layout of elements that are to be presented to a user by a user agent, such as, e.g., a Web browser. 
         [0043]    In HTML, a reference to any resource (which may be another HTML document, but need not be) may be comprised by an element called an “anchor”  136 . Each anchor is delimited by a start tag  137  and an end tag  138 . The start tag  137  comprises an element name  139  (which, for an anchor, is always “a”) and, as depicted in  FIG. 2 , an “href” attribute  141 , which is used to specify the target of the reference. The value  142  of the href attribute  141  is a Uniform Resource Locator (“URL”) which identifies the target for electronic retrieval and which implicitly or explicitly specifies the protocol by which the resource may be retrieved, the server from which the resource may be retrieved, and the path to the resource on the server. A start tag  137  for an anchor element  136  may comprise other attributes (not pictured) in addition to and/or instead of the href attribute  141 . 
         [0044]    Between the start tag  137  and the end tag  138  of an anchor element  136  is the body  143  of the element  136 . A Web browser will typically display only the body  143  of the element, but commonly will highlight or otherwise alter the appearance of text and/or other content within the body  143  to indicate that it is a hyperlink. When the user selects the hyperlink, e.g., by moving the pointer to the displayed body and then clicking the button on the pointing device, the Web browser attempts to retrieve the hyperlinked resource and present it to the user. 
         [0045]    As depicted in  FIG. 2 , the body  143  of the anchor element  136  is a citation. This kind of relationship is common in hyperlinked documents, such as, e.g., Web pages. The human-readable text indicates that another document is being referred to, while the enclosing markup provides information that an automatic system can use to retrieve the document for presentation to the user. This kind of relationship is merely a conventional usage, however, and it is well known in the art that the body  143  of an anchor element  136  need not be a human-readable reference or citation, but may be any text, image, and/or other content. 
         [0046]    An HTML document contains other tags besides those  137 ,  138  associated with anchor elements  136 . As depicted in  FIG. 2 , for example, the HTML fragment include tags  135  delimiting the beginning and end of a paragraph and other tags  140  indicating the beginning and end of a range of text that is to be presented to the user in an italic typeface. 
         [0047]      FIG. 3  depicts a small collection of documents, some of which refer to other documents. In connection with this example, the references may be considered machine-readable references, human-readable citations, or both. For example, as depicted in  FIG. 3 , document A  155  cites no other documents. In the legal context, sections of statutes, for example, often contain no citations. In contrast, document D  156  comprises a citation  157  to document A  155 , and document E  158  comprises a citation  159  to document A  155  and another citation  160  to document B  161 . 
         [0048]    Documents and their references may be considered elements of a directed graph, as that term is used in computer science, in which each node represents a document, and a reference is represented by an edge from the node representing the citing document to the node representing the cited document.  FIG. 4  depicts the documents and relationships of  FIG. 3  as a directed graph corresponding to a hypothetical collection of legal documents that cite each other. The documents may comprise any kind of legal source material, including, e.g., judicial opinions, statutes, regulations, treatises, articles in legal journals, etc. 
         [0049]    The graph of  FIG. 4  has no cycles. In other words, if one starts at any document in  FIG. 4 , one cannot follow the trail of citations back to the original document. This property may hold if all documents are, for example, judicial opinions, because an opinion is written at a specific time and, logically, may cite only earlier-published opinions. This property need not obtain for all types of documents, however. For example, secondary legal sources often cross-reference one another. For another example, a Web page may comprise a hyperlink to a second page that in turn comprises a hyperlink to the first page. Such cycles are common, e.g., within a Web site, when multiple pages link to one another as an aid to navigating the site. 
         [0050]    A reference from one document to another may suggest that the documents concern the same or related subjects. For example, as a matter of common practice, a hyperlink is inserted into a Web page to provide a link to a document that provides further information about something included within the referring page. For another example, in the legal field, an opinion or a treatise, for example, will typically cite at least one authority for every legal point it makes. Such an authority may be mandatory, such as a statute or a binding decision of an appellate court, or it may be persuasive, such as an opinion of a court in another jurisdiction. As a matter of good practice, though, the author of a document will typically cite the strongest, most relevant authority available for any particular point of law. 
         [0051]    Accordingly, in embodiments of the invention, references between documents are used to define clusters of documents that are likely to concern one or more related topics. A search space may then be constructed that is limited, in whole or in part, to one or more such clusters. (A collection of one or more references may be considered a definition of the search space comprising the referred-to documents.) 
         [0052]    In an embodiment of the invention, a search space is defined comprising a source document and all documents cited in the source document. For example, in connection with  FIG. 3 , if document E  158  is used as the source document, then the search space will comprise document E  158 , document A  155 , and document B  161 . If a search of the collection  150  is limited to this search space, then the results will comprise matching documents only from this subset, assuming any such matching documents exist. 
         [0053]    A search space can be defined iteratively, retrieving further cites from one or more cited documents. Starting with, for example, document G  163 , that document refers to document D  156 , document E  158 , and document F  162 , and these four documents may be regarded as the level 1 search space. Document D  156  in turn refers to document A  155 , and document E  158  refers to document B  161 , and therefore the level 2 search space consists of the documents in the level 1 search space plus document A  155  and document B  161 . 
         [0054]    A document may be encountered multiple times, and possibly on multiple levels, as the search space is constructed. For example, starting with document H  164 , the level 1 search space comprises document E  158  and document B  161 . Document E  158  refers to document B  161 , however, so document B  161  may be encountered again when constructing the level 2 search space from the level 1 search space. But since document B  161  has already been included in the level 1 search space, the further encounters are not significant and may be disregarded in an embodiment of the invention. 
         [0055]    In the general case, according to an embodiment of the invention, a search space of level N will consist of a source document and all documents that can be reached from it by following a chain of N or fewer references. To use the language of graph theory, a search space of level N will consist of the source document and all documents corresponding to nodes at a distance of N or less from the node corresponding to the source document, where each edge in the graph is directed and corresponds to a reference in one document to another. 
         [0056]      FIG. 5  depicts generating  180  a level N search space according to an embodiment of the invention. In block  185 , all references in a source document are identified. The way in which this is done may vary substantially depending on the nature and/or representation of the document and/or the design of the system that carries out this identification. 
         [0057]    In an embodiment of the invention, a source document may comprise references only in the form of human-readable citations, such as, e.g., the document  100  depicted in  FIG. 1 . In such an embodiment, identifying references  185  may comprise use of software programmed to recognize patterns of characters corresponding to known citation forms, possibly including commonly used standard and/or nonstandard variants. Such software may take a document as input and then store the found citations, e.g., in a data structure. 
         [0058]    In an embodiment of the invention, a source document may comprise computer-readable references, such as, e.g., those comprised by anchor elements  136  ( FIG. 2 ) in an HTML document  130  ( FIG. 2 ). XML may also be used to embed computer-readable references in a document in connection with an embodiment of the invention. Many other ways to encode computer-readable references are possible and will be apparent to those skilled in the art. In an embodiment of the invention, identifying references  185  may comprise use of software to read computer-readable references encoded in a document and then store the found references, e.g., in a data structure. 
         [0059]    In an embodiment of the invention, identifying references  185  in a document may take place substantially prior to use of that document in constructing a search space. For example, in an embodiment of the invention, the references in an document may be identified automatically when the document is first introduced to the system, when the document is indexed for use with a search engine, and/or when the document is revised, among other possibilities. In an embodiment of the invention, one or more references in a document may be identified by a human editor who inputs the reference or references into, e.g., a data structure. 
         [0060]    In an embodiment of the invention, some or all references may be normalized. Such normalization may, e.g., identify variant forms of references to the same document and thereby treat any occurrence of any variant as identical to any other occurrence of any variant. Normalizing references in this way may improve the efficiency of constructing the search space by limiting redundant processing of documents and/or references. 
         [0061]    The references so identified may be stored persistently, e.g., in a database management system, and then retrieved in connection with constructing  180  a search space. 
         [0062]    In an embodiment of the invention, a main iterative process is used to construct the search space, with the number of iterations being N, the desired level of the search space. In block  186 , a counter is set to indicate the current iteration. Retrieving the references from the source document may be considered the first iteration, so, as depicted in  FIG. 4 , the counter n is given the initial value of 1 in block  186 . Iteration proceeds until n equals N, and this condition is checked for in block  187 . 
         [0063]    In the depicted embodiment of the invention, constructing the level n+1 search space begins with all references defining the level n search space. This may be implemented by, for example, copying a data structure holding the references that define the level n search space to a new data structure that will hold the references that define the level n+1 search space, as in block  188 . 
         [0064]    Blocks  189 ,  190 , and  191  represent a subsidiary iterative process inside the main one. In block  189 , a check takes place that determines whether the references have been retrieved for every document in the level n search space. If not, references from the next unexamined document are retrieved in block  190 . In an embodiment of the invention, retrieval of references in block  190  may be similar to retrieval of references from the source document in block  185 . 
         [0065]    In block  191 , each reference that has been retrieved from the current document but is not yet part of the definition of the level n+1 search space is added to that definition. Flow then returns to block  189 , where a check for additional unexamined documents takes place. In an embodiment of the invention, this subsidiary process ends when the references have been retrieved for every document in the level n search space. 
         [0066]    Once references have been retrieved from all documents comprised by the definition of the level n search space, the counter n is incremented in block  192 , completing a single pass through the main iterative process. The check in block  187  is then repeated to determine if additional passes are necessary. If no additional passes are necessary, the definition of the level N search space is made available in block  193  for use in a search. 
         [0067]    In alternative embodiments of the invention, some references may be excluded from the definition of a search space, and/or some documents may not be examined for references. In some cases, for example, a reference may not be added because it is not recognized or because it refers to a document that is not indexed for searching. References may not be retrieved from a document because, for example, the document itself cannot be retrieved or, once retrieved, is in a form that cannot be examined for citations. 
         [0068]    In an embodiment of the invention, it may be possible to specify one or more classes of documents and/or references to be excluded from the process. For example, when used to construct a search space of legal documents, statutes and/or judicial opinions from foreign jurisdictions and/or references to such documents may be ignored in an embodiment of the invention. 
         [0069]    In an embodiment of the invention, information may be recorded about some or all relationships between and/or among documents in a search space, and such information may be used in connection with searches of the space. For example, the number of references in a first document to a second document may be recorded if it is believed that the number of references correlates with the degree of relevance between the two documents. In connection with a search space of level 2 or more, the level at which a document was added to the search space may be considered an indicator of the relevance of the document. Other properties of the reference graph, the documents, and/or the references may also be recorded, and, in an embodiment of the invention, some or all of the factors may be used to order the results of a search of any search space. 
         [0070]    Depending on, e.g., the nature of the documents, the size of the collection, and the ways in which documents in the collection refer to one another, it may be practical to define an exhaustive search space comprising every document directly or indirectly referred to in a source document.  FIG. 6  depicts defining  200  an exhaustive search space according to an embodiment of the invention. 
         [0071]    As before, defining  200  the exhaustive search space may in an embodiment of the invention be an iterative process that begins in block  201  with retrieving the references found in a source document. Because the search is exhaustive, it ends when no references can be found besides those already in the definition of the search space. This terminating condition is checked for in block  202 . 
         [0072]    In blocks  203 ,  204 , and  205 , the references from each of the documents added to the search space in the previous pass are added to the definition of the search space. The determination is made in block  203  whether any documents remain for which the references have to be added. If so, the references from the next document are retrieved in block  204  and added to the definition of the search set in block  205 . The check in block  203  is then repeated. 
         [0073]    Once references have been retrieved from each document added during the previous pass, the check in block  202  is repeated. If the latest pass did not increase the size of the search space, then the exhaustive search space has been successfully defined, and, in block  206 , its definition is made available for use in searching. 
         [0074]    In an embodiment of the invention, the definition  200  of the exhaustive search space may be interrupted if the process exceeds a preset running time and/or if the search space exceeds a preset size. 
         [0075]    So far, the invention has been discussed in connection with an embodiment that builds a search space by moving from referring documents to referenced documents, but the invention is not limited to such embodiments. In embodiments of the invention, a document collection may be indexed so that references between documents are stored, e.g., in a database. In such an embodiment, retrieving the references from a document may comprise submitting a query to the database that requests all references comprised by the source document. 
         [0076]    In another such embodiment, however, it is possible to traverse the graph in the opposite direction, e.g., by submitting a database query that identifies all documents that refer to a source document. Referring to  FIG. 3  and taking, for example, document B  161  as the source document, the level 1 search space comprises document B  161 , document E  158 , and document H  164 . The level 2 search space additionally comprises document G  163 . 
         [0077]    Depending on the embodiment of the invention and the nature of the searchable data collection, it may be impossible, impractical, or undesirable to identify all documents that hold references to a particular document. For example, identifying all Web pages that link to any particular page may not be practical, but such references may be recorded as individual pages are indexed for search. Such indexing of web references is done by, for example, the Google™ Internet search engine. 
         [0078]    In a legal context, a search space so constructed may comprise, e.g., lines of decisions and/or analysis based on a significant statute or judicial opinion. 
         [0079]    In an embodiment of the invention, a search space may be defined by traversing the graph in either or both directions. In some further embodiments of the invention, a user may choose the type of search space desired. 
         [0080]    It will be appreciated by those skilled in the art that there are many ways to implement searching in a search space such as is built in an embodiment of the invention. For example, tools for searching within collections of textual data are well known in the art and are available from commercial suppliers. One such tool is Oracle® Text, which supports text searching combined with ordinary database searching in a single SQL statement. 
         [0081]    Thus, in an embodiment of the invention, a relational database management system comprises one or more documents stored so as to allow each document to be associated with metadata. After a search space is constructed, the metadata associated with each document in the search space is modified to indicate such membership. (In an embodiment of the invention, a document can be included in multiple search spaces simultaneously.) When a search query is received for a given search space, the query is transformed into a SQL statement that specifies a textual search, limited to documents already identified in their associated metadata as part of the search space. 
         [0082]    In an alternative embodiment of the invention, a search engine conducts a search as if no topical search space as described herein has been specified, and then uses the definition of the desired search space to filter the results. Filtering search results according to such an embodiment of the invention is depicted in  FIG. 7 . In block  220 , a user selects a topical area for the search. The nature of this selection will depend on the embodiment of the invention. In one embodiment, however, a user is presented with a hierarchical directory of topics and subtopics. By appropriate navigation, the user may select one or more topics relevant to the search. 
         [0083]    In block  221 , the user specifies the criteria for the search by entering a query. Many types of search queries are well known in the relevant arts, and include, for example, Boolean queries and natural-language based queries. In block  222 , a search is performed across the entire document collection, producing, e.g., a list of results. 
         [0084]    In block  223 , the definition of the search space is retrieved, and in block  224 , this definition is used to filter the results of the search from block  222 . In an embodiment of the invention, the definition of the search space comprises normalized citations to every document comprised by the search space. Filtering the result in block  224  may comprise, for example, checking every document in the list of results against the definition of the search space and removing from the results any documents not found in that definition. Finally, in block  225 , the filtered results of the search are presented to the user, who may then, in an embodiment of the invention, be able to retrieve and work with some or all of the documents comprised by the results. 
         [0085]    The contents of the search space may be expected to depend heavily on the contents of the source document used to generate it. In an embodiment of the invention, a set of topical source documents is used as the basis for generating topical search spaces. Such a topical document (which may be referred to as a “law note”) may, for example, be prepared by an expert in the field and may cite those sources judged to be most relevant to the topic. 
         [0086]    For example, a particular law note may be a memorandum or article discussing, e.g., the application of principles of securities fraud to trading of debt securities by persons having material nonpublic information. Another law note may, for example, comprise a compilation of citations to laws and/or regulations regarding a particular topic, e.g., securities regulation. Another law note may be a directory of authorities judged particularly relevant to, e.g., securities regulation. Each such document may be written and maintained by one or more experts in securities law, and may cite the relevant statutory provisions and regulations and/or leading case law. Depending on the purpose for which the document was written, it may also cite many court opinions reflecting application of the principles discussed in the other authorities. In an embodiment of the invention, this article may be used as the source article in the generation of a search space when a user is looking for information about, e.g., insider trading of corporate bonds. 
         [0087]    In an embodiment of the invention, a user may select a topic for searching from a screen  250  such as depicted in  FIG. 8 . A caption  255  may be present, indicating, e.g., the function of display, the collection of documents, and/or other information. The display may comprise, e.g., one or more menus  256  and/or other means for selecting one or more commands and/or additional functions. 
         [0088]    One or more high-level topics may be displayed, such as, e.g., Environmental Law  257 , Family Law  258 , and Insurance Law  259 . The organization of topics may be hierarchical, with one or more topics associated with one or more sub-topics, which may in turn be associated with one or more further sub-topics, and so on. (For ease of description “topic” comprises “sub-topic” as used herein.) For example, as depicted in  FIG. 8 , the topic Securities Law  260  is associated with the topic Securities Regulation  261 , which is in turn associated with the topics Federal Regulation of Securities  262 , State Regulation of Securities  263 , Exchange &amp; SRO Regulation  264 , and International Regulation of Securities  265 . In an embodiment of the invention, the organization may be polyarchical, in which any particular topic may be associated with more than one higher-level topic. 
         [0089]    A tree  270  may be used to present the hierarchically-organized topics. An icon  271  may indicate the presence of one or more topics subsumed by a topic or topic. Icons  272  may also be provided that, when selected, cause all topics below a particular topic to be hidden. 
         [0090]    Selection  273  of one or more topics and/or sub-topics may be reflected in the tree  270  by highlighting and/or other visual means. When a topic  263  is selected in the tree  270 , names and/or titles  274  of one or more documents and/or directories of documents may be presented in a document area  275 . In an embodiment of the invention, one or more law notes may be identified in the document area  275 . 
         [0091]    It is possible in an embodiment of the invention for a user to select multiple topics and/or subtopics to be searched simultaneously. In such an embodiment, the search space may be the union of the search spaces corresponding to each of the selected topics. For example, selecting the topic Securities Regulation  261  would result in a search space comprising the union of the search spaces corresponding to Federal Regulation of Securities  262 , State Regulation of Securities  263 , Exchange &amp; SRO Regulation  264 , and International Regulation of Securities  265 . In an embodiment of the invention, multiple discrete items may be selected at the same time, e.g., General Practice  259  and State Regulation of Securities  265 . 
         [0092]    A search function may be provided, and  FIG. 9  depicts a display  290  from which a user may enter a search query in an embodiment of the invention. A text area  291  may be present, displaying a query as it is entered and/or edited. One or more controls  292  may also be present, allowing the search to be restricted, e.g., by date, document type, jurisdiction, and/or other criteria. 
         [0093]    A control  293  may allow the user to select the scope of the search. For example, the search may be limited to the text of one or more documents presented in the document area  275  ( FIG. 8 ). Alternatively, a search may be specified of a search space defined in accordance with the invention, as described above. In an embodiment of the invention, the source document used to construct the search space may be one or more law notes associated with the selected topic or topics and/or any sub-topics comprised by the one or more sub-topics. 
         [0094]    In an embodiment of the invention, the user may be provided with one or more controls (not pictured) allowing specification of the level of search space to be used. In another embodiment of the invention, the level associated with some or all topics and/or source documents may be fixed. For example, it may be determined that, when a source document is a law note that comprises compiled references to statutes and regulations, a level-3 search space provides optimum coverage of the topic, which may mean, e.g., that such a search space is most likely to incorporate relevant material without overinclusion. (The use of a level-3 search space in this example is meant to be purely exemplary, and should not be taken to limit the invention, even when the source document is a law note as described herein.) 
         [0095]    The invention has been described above in connection with certain preferred embodiments. This description is purely illustrative and not limiting, and other embodiments of the invention will be apparent to those skilled in the relevant arts.