test whether two lists intersect
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.