Question: What are the two types of transformation rules?
Evidence: The same procedure of generating a single triple can be applied for the generation of each triple in a set of triples. However, the NL output would contain redundant information and consequently sound very artificial. Thus, the goal is to transform the generated description to sound more natural. To this end, we focus on two types of transformation rules (cf. BIBREF21): ordering and clustering and grouping. In the following, we describe the transformation rules we employ in more detail. Note that clustering and ordering (SECREF18) is applied before grouping (SECREF26).
We process the input trees in descending order with respect to the frequency of the variables they contain, starting with the projection variables and only after that turning to other variables. As an example, consider the following triples about two of the most known people in the world:
The five triples are verbalized as given in UNKREF20–UNKREF24. Clustering and ordering first take all sentences containing the subject :Albert_Einstein, i.e. UNKREF21 –UNKREF23, which are ordered such that copulative sentences (such as Albert Einstein is a scientist) come before other sentences, and then takes all sentences containing the remaining subject :William_Shakespeare in UNKREF20 and UNKREF24 resulting in a sequence of sentences as in UNKREF25.
William Shakespeare is a writer.
Albert Einstein's birth place is Ulm.
Albert Einstein's death place is Princeton.
Albert Einstein is a scientist.
William Shakespeare's death date is 23 April 1616.
Albert Einstein is a scientist. Albert Einstein's birth place is Ulm. Albert Einstein's death place is Princeton. William Shakespeare's is a writer. William Shakespeare's death date is 23 April 1616.
dalianishovy describe grouping as a process “collecting clauses with common elements and then collapsing the common elements”. The common elements are usually subject noun phrases and verb phrases (verbs together with object noun phrases), leading to subject grouping and object grouping. To maximize the grouping effects, we collapse common prefixes and suffixes of sentences, irrespective of whether they are full subject noun phrases or complete verb phrases. In the following we use $X_1,X_2,$...$X_N$ as variables for the root nodes of the input sentences and $Y$ as variable for the root node of the output sentence. Furthermore, we abbreviate a subject $\texttt {subj}(X_i,s_i)$ as $\texttt {s}_i$, an object $\texttt {dobj}(X_i,o_i)$ as $\texttt {o}_i$, and a verb $\texttt {root}(\text{\em ROOT}_i,v_i)$ as $\texttt {v}_i$.
Subject grouping collapses the predicates (i.e. verb and object) of two sentences if their subjects are the same, as specified in UNKREF27 (abbreviations as above).
$\rho (\texttt {s}_1)=\rho (\texttt {s}_2)\wedge \texttt {cc}(\texttt {v}_1,\text{\em coord})$
$\Rightarrow \texttt {root}(Y,\texttt {coord}(\texttt {v}_1,\texttt {v}_2))\wedge \, \texttt {subj}(\texttt {v}_1,\texttt {s}_1)\wedge \texttt {dobj}(\texttt {v}_1,\texttt {o}_1)\wedge \texttt {subj}(\texttt {v}_2,\texttt {s}_1)\wedge \texttt {dobj}(\texttt {v}_1,\texttt {o}_2) $
An example are the sentences given in UNKREF28, which share the subject Albert Einstein and thus can be collaped into a single sentence.
Albert Einstein is a scientist and Albert Einstein is known for general relativity.
$\Rightarrow $ Albert Einstein is a scientist and known for general relativity.
Object grouping collapses the subjects of two sentences if the realizations of the verbs and objects of the sentences are the same, where the $\text{\em coord}\in \lbrace \text{\sf and},\text{\sf or}\rbrace $ is the coordination combining the input sentences $X_1$ and $X_2$, and $\texttt {coord}\in \lbrace \texttt {conj},\texttt {disj}\rbrace $ is the corresponding coordination combining the subjects.
$\rho (\texttt {o}_1)=\rho (\texttt {o}_2)\wedge \rho (\texttt {v}_1)=\rho (\texttt {v}_2)\wedge \texttt {cc}(\texttt {v}_1,\text{\em coord})$
$\Rightarrow \texttt {root}(Y,\texttt {PLURAL}(\texttt {v}_1))\wedge \, \texttt {subj}(\texttt {v}_1,\texttt {coord}(\texttt {s}_1,\texttt {s}_2))\wedge \texttt {dobj}(\texttt {v}_1,\texttt {o}_1) $
For example, the sentences in UNKREF30 share their verb and object, thus they can be collapsed into a single sentence. Note that to this end the singular auxiliary was needs to be transformed into its plural form were.
Benjamin Franklin was born in Boston. Leonard Nimoy was born in Boston. $\Rightarrow $ Benjamin Franklin and Leonard Nimoy were born in Boston.
To this end, we focus on two types of transformation rules (cf. (Dalianis and Hovy, 1996)): ordering and clustering and grouping.
Answer: ordering and clustering and grouping.