Returns the hash code value for this set. The hash code of a set is
defined to be the sum of the hash codes of the elements in the set,
where the hash code of a null element is defined to be zero.
This ensures that s1.equals(s2) implies that
s1.hashCode()==s2.hashCode() for any two sets s1
and s2, as required by the general contract of
Object.hashCode.
Object.equals(Object), Set.equals(Object)
Diagram: Collection Set