There's a flag Declaration.IS_SINGLE_VALUE, but I don't understand how
that interacts with type inference or code generation.
I did a little experiment: introduce a subclass of SpecialObjectType
named SingleValueType. That should mostly be like java.lang.Object but
not compatible with gnu.mapping.Values. Values instances would require
special coercion code. Anyway, that experiment didn't go far.
So should I continue with the SingleValueType idea or is there a better
way to do it?