SUBST-IF
========

syntax: (subst-if <new> <predicate>
             <tree>)                 ; Common Lisp
        (tree-substitute-if <new> <predicate>
             <tree>)                 ; Babylon Lisp

This function exchanges the elements, which have the
right predicate, against new ones. It does not only
work on the list <tree>, but it works too on all
lists, which are included in <tree>.

example:    >(subst-if 'c #'numberp '(a b 1 (1 b a)))
            (A B C (C B A))