Used to create different datatypes each time the functor is applied

Value returned when a map is queried, and the base is not present. `Bottom indicates that the base is never bound in such a map.

This function is used to optimize functions that add keys in a map, in particular when maintaining canonicity w.r.t. default contents. It describes the contents c of the offsetmap resulting from default_offsetmap b. The possible values are:

• Bottom means that c is V.bottom everywhere, and furthermore that V.bottom has an empty concretization. We deduce from this fact that unmapped keys do not contribute to a join, and that join c v is never c as soon as v is not itself v.
• Top means that c is V.top everywhere. Thus unmapped keys have a default value more general than the one in a map where the key is bound.
• `Constant v means that c is an offsetmap with a single interval containing v everywhere. v must be isotropic (in the sense of V.is_isotropic).
• `Other means that default_offsetmap returns something arbitrary.

This function is only used on keys that change values. Thus it is safe to have default_offsetmap return something that do not match default_contents on constant keys.