// snlMatch2 keeps existing matches and restores matches on false

static boolean snlMatch2(S pat, S snl, SNLMatches matches) {

  ret snlMatch2(snlToTree_cached(pat), snlToTree(snl), matches);

}

static boolean snlMatch2(Lisp pat, S snl, SNLMatches matches) {

  ret snlMatch2(pat, snlToTree(snl), matches);

}

static boolean snlMatch2(S pat, Lisp snl, SNLMatches matches) {

  ret snlMatch2(snlToTree_cached(pat), snl, matches);

}

static boolean snlMatch2(Lisp pat, Lisp snl, SNLMatches matches) {

  int level = matches.save();

  if (snlMatch2_sub(pat, snl, matches))

    ret true;

  matches.restore(level);

  ret false;

}

static boolean snlMatch2_sub(Lisp pat, Lisp snl, SNLMatches m) {

  if (pat == null || snl == null) ret false;

  if (pat.isA("*"))

    ret true;

  if (startsWithUpperCase(pat.head))

    ret snlMatch2_putMatch(m, pat.head, snl);

  if (neq(pat.head, snl.head)) ret false;

  // heads identical, proceed to children

  int n = pat.size();

  if (n != snl.size()) ret false;

  for (int i = 0; i < n; i++)

    if (!snlMatch2_sub(pat.get(i), snl.get(i), m))

      ret false;

  ret true;

}

static boolean snlMatch2_putMatch(SNLMatches matches, S key, Lisp val) {

  ret matches.put(key, val);

}