(Intersectp-equal x y) has a guard that x and y are true lists.
Major Section: PROGRAMMING
(Intersectp-equal x y) returns t if x and y have a
member in common, else it returns nil. Also
see intersectp-eq, which is logically the same but can be more
efficient since it uses eq instead of equal to look for
members common to the two given lists.
(Intersectp-equal x y) has a guard that x and y are true lists.