The 'cond' special form evaluates a series of 
(cond                                       ; sample CONDitional
  ((not T) (print "this won't print"))
  ( NIL    (print "neither will this"))
  ( T      (print "this will print"))
  ( T      (print "won't get here")))       ; prints "this will print"
(defun print-what (parm)
  (cond                                     ; start of COND
    ((numberp parm) (print "numeric"))      ; check for number
    ((consp parm)   (print "list"))         ; check for list
    ((null parm)    (print "nil"))          ; check for NIL
    (T              (print "something"))))  ; catch-all
(print-what 'a)       ; prints "something"
(print-what 12)       ; prints "numeric"
(print-what NIL)      ; prints "nil"
(print-what '(a b))   ; prints "list"
See the
cond
special form in the