MEMBERP

(MEMBERP <what> <list> [<test>])

If <what> is a member of <list> (in the sence of EQL), returns true.
Othervise, returns false. If the <list> is dotted, the value after the
dot is silently ignored. 

If the value of <test> is given, it must be a predicate of two arguments, 
first of them for <what>, and the second is for an item of the <list>; 
it will be used instead of EQL.

NOTE this function works much like MEMBER (in fact, it shares its 
implementation) with the only difference that MEMBERP always returns 
either True or False, not a tail of the list.