static Map<S, Lisp> lispMatch(Lisp pat, Lisp nl, Collection<S> variables) {
  new Map<S, Lisp> matches;
  ret lispMatch_sub(pat, nl, variables, matches) ? matches : null;
}

static bool lispMatch_sub(Lisp pat, Lisp nl, Collection<S> variables, Map<S, Lisp> m) {
  if (pat == null || nl == null) ret false;
  if (pat.isLeaf() && variables.contains(pat.head))
    ret lispMatch_putMatch(m, pat.head, nl);
    
  if (neq(pat.head, nl.head)) ret false;
  
  // heads identical, proceed to children
  int n = pat.size();
  if (n != nl.size()) ret false;
  
  for (int i = 0; i < n; i++)
    if (!lispMatch_sub(pat.get(i), nl.get(i), variables, m))
      ret false;
      
  ret true;
}

static bool lispMatch_putMatch(Map<S, Lisp> matches, S key, Lisp val) {
  if (matches.containsKey(key)) {
    if (neq(matches.get(key), val))
      false;
      //fail("multi-matching not implemented");
    matches.put(key, val);
  }
  true;
}