INTERSECTP

test whether two lists intersect
Major Section:  PROGRAMMING

General Forms:
(set-difference$ l1 l2)
(set-difference$ l1 l2 :test 'eql)   ; same as above (eql as equality test)
(set-difference$ l1 l2 :test 'eq)    ; same, but eq is equality test
(set-difference$ l1 l2 :test 'equal) ; same, but equal is equality test

(Intersectp l1 l2) returns t if l1 and l2 have a member in common, else it returns nil. The optional keyword, :TEST, has no effect logically, but provides the test (default eql) used for comparing members of the two lists.

The guard for a call of intersectp depends on the test. In all cases, both arguments must satisfy true-listp. If the test is eql, then one of the arguments must satisfy eqlable-listp. If the test is eq, then one of the arguments must satisfy symbol-listp.

See equality-variants for a discussion of the relation between intersectp and its variants:

(intersectp-eq x lst) is equivalent to (intersectp x lst :test 'eq);

(intersectp-equal x lst) is equivalent to (intersectp x lst :test 'equal).

In particular, reasoning about any of these primitives reduces to reasoning about the function intersectp-equal.