Bison *must* be optional.
why *must* it be optional? arguments about "standard tools" fall flat because a standard win 32 system lacks a compiler anyway... There are many bison implementations, including win 32 or dos versions.
As was mentioned bison is one of the most standard compiler generation tools... available from many places, including installed as a standard package in many linux distribs.
i believe in make things work first, then make them better.
my 0.02...
Youness