Sometimes, however, the same document conforms to several overlapping structures, and its processing varies according to the particular structure considered. A Shakespearean play, for example, can be viewed as a structure of acts, each of which is made up of scenes, each of which is in turn composed of speeches and stage directions. But it is also sometimes useful to regard scenes as collections of verse lines. These two views of a scene cannot be fitted into the same hierarchical structure: speeches may contain several lines of verse, but lines of verse can also contain more than one speech. […] As we noted above, this kind of non-hierarchical structure constitutes a problem for an SGML-based encoding system for literary texts: SGML is not designed to accommodate overlapping structural elements.

(Barnard et al. 1988, 266-267)

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