Clearly, there are many such trees that might be drawn to describe the structure of this or other anthologies. But equally clearly there are many other trees that might be drawn which do not fit within this tree. Or, to take a simpler example, we might want to represent the pagination of different editions of the same text. One way of doing this would be to group the lines and titles of our current model into pages. That is, a page consists of one or more unnamed groups, each of which contains an optional title, followed by a sequence of lines.
Some poems are longer than a single page, and other pages contain more than one poem.
A separate document type definition must be created for each hierarchic tree into which the text is to be structured. As this example shows, the name of a document type must always be the same as the name of the largest element in it, that is the element at the top of the hierarchy. To mark up the two views, it will be necessary to indicate which hierarchy each element belongs to. As a simple example, let us assume that Blake's poem appears in some paged anthology, with the page break occurring half way through the first stanza. O Rose thou art sick.