Definition 0.3. A partial ordering on a set S is a relation that is reflexive and transitive, and such that if a < b and b < a then a = b. Given a partial order, < on a set S for x,y E S we say that S is partially ordered by <. A pair S, < is called a partially ordered set Definition 0.4. A total ordering on a set S is a partial order 3 on S such that given a,b ES, a < b or b