Module Base__.Commutative_group

A signature for a commutative group (in the group-theory sense).

An implementation of this interface should have the following properties:

1. associativity: (a+b)+c = a+(b+c) for all elt's a,b,c
2. identity: zero+a = a+zero = a for all elt's a
3. inverses: given any elt a there exists a (unique) elt b such that a+b=b+a=zero
4. commutativity: a+b = b+a