Unnecessary checks.
It seems 2-3-4-trees are working.
Implemented IntersectWith().
Implemented IsProperSubsetOf().
Implemented IsSubsetOf()
Implemented IsSupersetOf() and IsProperSupersetOf().
Implemented Overlaps().
Moved null check to base class.
Implemented SetEquals().
Added RemoveRange().
Using Distinct() in Overlaps().
Implemented SymmetricExceptWith().