Yacc neophyte vs inscrutable error message

[Mark Bartelt]

OK, yaccers, here's a quickie question.  I have a grammar that causes
UNIX yacc (on a VAX, running 4.3bsd) to choke.  It bombs out with the
following error message:

 fatal error: default action causes potential type clash, line xxx

Curiously, the very same grammar is accepted without complaint by MKS's
yacc for MS-DOS, which (as far as I can tell) produces code that does
what it's supposed to do.

I've RTFMed, but the FM isn't terribly helpful here.  I've also looked
briefly at the source code, which, to put it mildly, is a bit cryptic,
at least for someone unfamiliar with the internals of yacc.

I doubt that anyone wants to see the entire 300 or so lines of my yacc
grammar, but can anyone provide a brief explanation of what yacc may
be complaining about, and what the error message really means?  And,
for that matter, why the UNIX yacc complains about something that's
quite acceptable to the MKS yacc?  Is it likely that I've botched the
grammar, or is it possible that there's a bug in 4.3bsd yacc?  Or is
the MKS yacc at fault for not catching something that the 4.3bsd yacc
properly complains about?

Thanks in advance.

-----

Mark Bartelt                          UUCP: {utzoo,decvax}!sickkids!mark
Hospital for Sick Children, Toronto   BITNET: mark at sickkids.utoronto
416/598-6442                          INTERNET: mark at sickkids.toronto.edu