A partial ordering on sets: http://en.wikipedia.org/wiki/Partially_ordered_set
This can be defined using either |joinLeq| or |meetLeq|, or a more efficient definition can be derived directly.
The superclass equality (which can be defined using |partialOrdEq|) must obey these laws:
Reflexive: a == a Transitive: a == b && b == c ==> a == b
The equality relation induced by the partial-order structure
Fixed points of chains in partial orders
Least point of a partially ordered monotone function. Checks that the function is monotone.
Least point of a partially ordered monotone function. Does not checks that the function is monotone.
Greatest fixed point of a partially ordered antinone function. Checks that the function is antinone.