Before this fix, equiv_induct only assumed that one of the following is
true:
- defined value of A is equal to defined value of B
- A is undefined
This lets through valuations where A is defined, B is undefined, and
the defined (meaningless) value of B happens to match the defined value
of A. Instead, tighten this up to OR of the following:
- defined value of A is equal to defined value of B, and B is not
undefined
- A is undefined
o Not all derived methods were marked 'override', but it is a great
feature of C++11 that we should make use of.
o While at it: touched header files got a -*- c++ -*- for emacs to
provide support for that language.
o use YS_OVERRIDE for all override keywords (though we should probably
use the plain keyword going forward now that C++11 is established)