TREE-SUBSTITUTE
===============

syntax: (tree-substitute <new> <old> <tree>
              [:test <test>])         ;Babylon Lisp
        (subst <new> <old> <tree> 
              [:test <test>])         ;Common Lisp

This function exchanges the old elements of the list
<tree> 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:       >(tree-substitute 12 1 '((21 1) 3 1))
               ((21 12) 3 12)