[developers] Finding the minimal type for which a feature is declared

[Ann Copestake]

type-ancestors (now called ltype-ancestors) stores a list of all supertypes of
a given type.  This is what's used for checking whether one type is a subtype
of the other (see fns in main/types.lsp).  I can't quite see the circumstances
under which it would be better to store a single list.  The maximum types for
individual features are cached - I can imagine that there are faster
implementations of maximal-type-of-list, which proceeds by doing successive
binary glb calculations on the maximal types for each feature, but the glb
calculations are very optimised and I doubt the time taken is very significant.

Ann