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