Complement

Complement[alle1e2, ...]
returns an expression containing the elements in the set all that are not in any of e1, e2, etc.
Complement[alle1e2, ..., SameTest->test]
applies test to the elements in all and each of the ei to determine equality.

The sets all, e1, etc can have any head, which must all match. The returned expression has the same head as the input expressions.