DTD problem: transforming cyclic graph into element definitions

[Yves Forkl]

I need to transform a directed cyclic graph into a DTD. My approach is
to have one element Pn for each "production" number n, with the
content model comprising the starting vertex number n and the
subsequent productions. For e.g., for vertex V1 and its edges leading
to V4 and V7, I would get:

<!ELEMENT P1 (V1, (P4 | P7)) >

For the following part of the graph I can't come up with the two
content models because I always run into illegal recursive structures:

Pa: Va -> Va | Vb | Vc
Pb: Vb -> Va | Vc

Maybe someone has an suggestion for legally recursive content models
for Pa and Pb?

Yves