In case anyone is interested in the somewhat obscure problem "how could I generalize the concept of interning and sharing unique DAGs so that I can allow the interned graphs to contain cycles?" or in any other formulation of the graph minimization problem described in Guido Tack's online notes, feel free to check out my latest attempt to solve the problem as a Common Lisp prototype program, with an unreviewed preprint describing the basic algorithm.
(The problem comes up in various fields. One example is detecting the equivalence of arbitrarily complex cyclic types in programming languages. Nonincremental minimization algorithms have been known for a long time, I'm trying to do it incrementally.)