There is no question but that most of our textual archive is hierarchically organized. On the other hand, there is also no question but that poetical texts comprise a key, perhaps even a defining, part of the corpus of our humanities archive. When Plato called for the expulsion of the poets from the city, he was arguing for a certain theory of textuality. Unlike expository text, poetry is not organized in a determinate hierarchy. TEI and SGML markup, therefore, while reasonably adequate vehicles for expository and informational texts, fails to render those features of poetic text that are most salient for its makers and users.
Poetical texts are recursive structures built out of complex networks of repetition and variation.
Of course a governing hierarchy can be imposed upon such works. The case of poetry in fact defines a kind of textual ethos, as it were, that may be seen to pervade genres not normally thought of as poetical. Reference Renear, Allen.