The endomorphic fragment of a Functor: the structural shape cannot speak the
family's own re-instantiation at a new value type, so it captures map at one
value type — enough to state the functor laws (identity and composition),
proven per family in laws.spec.
The endomorphic fragment of a Functor: the structural shape cannot speak the family's own re-instantiation at a new value type, so it captures map at one value type — enough to state the functor laws (identity and composition), proven per family in laws.spec.