E - Element typepublic static interface PartiallyOrderedSet.Ordering<E>
To obey the constraints of the partially-ordered set, the function must be consistent with the reflexive, anti-symmetric, and transitive properties required by a partially ordered set.
For instance, if ordering(foo, foo) returned false for any
 not-null value of foo, it would violate the reflexive property.
If an ordering violates any of these required properties, the behavior
 of a PartiallyOrderedSet is unspecified. (But mayhem is
 likely.)