A predicate in Lisp is a function that performs a test and returns either T (True) or NIL (False). Note: Lisp functions that test predicate values consider NIL to be False and anything else to be True. Predicate names often end in P. Some commonly used predicates are:
(SYMBOLP x) True if x is a SYMBOL.
(ATOM x) True if x is an ATOM
(anything besides a CONS).
(NULL x) True if x is NIL.
(CONSP x) True if x is a CONS cell.
(NUMBERP x) True if x is a number.
(ZEROP x) True if x is zero;
error if x not a number.
(MINUSP x) True if x is negative;
error if x not a number.