The :default value is t,
because constructs like identifiers and constants are unambiguous;
ambiguities may only exist in expressions, type names, etc.
We override the boilerplate to return nil on
the fixtype cases with ambig in their names,
the fixtypes amb-...,
and the dummy base case of dirabsdeclor;
although the latter is not properly an ambiguous construct,
we take the opportunity to exclude it here from consideration
whenever unambiguous constructs are concerned
(which is for most of the tools, except for parser and disambiguator).
We override the boilerplate to return t on
GCC attributes, attribute specifiers, and assembler constructs.
