Applies conversions to the two arguments of a binary operator.
DESCRIPTION
If c1 is a conversion where c1 `l` returns |- l = l' and c2 is a
conversion where c2 `r` returns |- r = r', then
BINOP2_CONV c1 c2 `op l r` returns |- op l r = op l' r'. The
term op is arbitrary, but is often a constant such as addition or
conjunction.
FAILURE CONDITIONS
Never fails when applied to the conversion. But may fail when applied to the
term if one of the core conversions fails or returns an inappropriate theorem
on the subterms.