Category of Abelian monoids
Cat::AbelianMonoid represents an Abelian
Cat::AbelianMonoid is an Abelian semi-group
with a neutral element
dom::zero according to the
Use the axiom
state that zero is always represented in a unique way (i.e. canonically).
If an Abelian monoid has not the axion
only one possible representation of the neutral element. An Abelian
semi-group must at least have the method
test for zero in such a case.
Must hold the neutral element according to the operation