Reality check -- card catalogs don't support graph structure?

[Nick Arnett]

For the book I'm working on, I'm making some comparisons between hypertext
catalogs and libraries that use standard cataloging, such as Dewey and LOC.
 Am I correct in saying that the standard catalogs do not contain graph
structures?  That is to say, every subheading has just one parent heading?
Another way to put this is that a subheading cannot appear in more than one
place in the catalog.  Yet another way to say this is that typical library
catalogs are trees, whereas Yahoo! and catalogs like it are graphs.

I'd also be curious if there has been any effort to bring graph structures
into the established cataloging systems.

This is going into a section on enabling serendipity.  Graph structures
support serendipity in more dimensions than trees, but were unworkable as
long as catalogs were stored on cards.

Nick