sbool lispMatch_debug;

static Map<S, Lisp> lispMatch(Lisp pat, Lisp input, S... variables) {
  ret lispMatch(pat, input, asSet(variables));
}

static Map<S, Lisp> lispMatch(Lisp pat, Lisp input, Collection<S> variables) {
  new Map<S, Lisp> matches;
  ret lispMatch_sub(pat, input, 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)) {
    if (lispMatch_debug) print("Var: " + pat.head + " => " + nl);
    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_debug) print("Sub " + i + ": " + pat.get(i) + " => " + nl.get(i));
    if (!lispMatch_sub(pat.get(i), nl.get(i), variables, m))
      false;
  }
      
  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");
  } else
    matches.put(key, val);
  true;
}

Began life as a copy of #1002842