S9fES  (gensym string)  ==>  symbol
       (gensym)         ==>  symbol

Return a fresh symbol that is guaranteed to be unique. When STRING
is passed to GENSYM, the returned symbol will have this string as
a prefix.

(gensym)        ==>  g56
(gensym "foo")  ==>  foo57

(let ((a (gensym))
      (b (gensym))
      (c (gensym)))
  (list a b c))      ==>  (g63 g62 g61)
