alg for "compacting" series-parallel graphs (yacc->BNF)?

Does anyone know of an algorithm to find a "compact representation" of
a series-parallel graph? Eg


      -- a -- b -- c -- should -- b --
    / \ become / \
----- a ------- c ----- -- a ----------- c --


The set of all paths from source to target should remain the same,
while the number of nodes should be minimized.


I want to use this for a convertor from yacc to BNF. Yacc only allows
top-level alternatives while BNF also allows grouping
    x : a b c x : a (b | ) c
        | a c


Also, how can I use the looping constructs of EBNF? I think I know how
to do it for the most-often used cases
    xs : x and xs :
          | xs ',' x | xs ',' x
but what would be a general algorithm?


All this is for a yacc-grammar prettyprinter, which will take a
y.output file and produce a railroad diagram using rail.sty or
syntax.sty (available on CPAN).


Please CC me if you reply; I will summarize the replies.
TIA, Vlad




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