intersection on relations is bugged

[pfdietz]

(equalp (intersection (convert '2-relation '((1 . 2))) (convert '2-relation '((1 . 3)))) (empty-2-relation)) ==> NIL

The problem is the representation for the intersection maps 1 to an empty set, but this pair should have been removed from the tree, as it causes EQUAL? to indicate they are not the same.