This class lets us deal with the units in a ring. isUnit tells whether an element is a unit. The other operations let us canonically write an element as a unit times another element. Two elements a, b of a ring R are _associates_ if a=b*u for a unit u. For an element a, we want to write it as a=b*u where b is an associate of a. The map (a->b) is called StandardAssociate by Gap, "unitCanonical" by Axiom, and "canAssoc" by DoCon. The map (a->u) is called "canInv" by DoCon and "unitNormal(x).unit" by Axiom.

The laws are

  stdAssociate x * stdUnit x === x
    stdUnit x * stdUnitInv x === 1
 isUnit u ==> stdAssociate x === stdAssociate (x*u)

Currently some algorithms assume

 stdAssociate(x*y) === stdAssociate x * stdAssociate y

Minimal definition: isUnit and (stdUnit or stdUnitInv) and optionally stdAssociate

Currently some algorithms assume this property.