PPT Slide
Instead of returning T or NIL, some predicates in Common Lisp return NIL for false and any nonNIL value for true.
Member determines if a value is an element of a list.
NIL if X is not a top-level element of list L
L’ if X is a top-level element of L and L’ is the sublist starting with X and having all the remaining elements of L.
( member ’a ’( b c d a e ) )