static boolean jmatchStart(S pat, S s) {
    return jmatchStart(pat, s, null);
  }
  
  // matches are as you expect, plus an extra item for the rest string
  static boolean jmatchStart(S pat, S s, Matches matches) {
    if (s == null) return false;
    L<S> tokpat = javaTok(pat), toks = javaTok(s);
    if (toks.size() < tokpat.size()) ret false;
    S[] m = match2(tokpat, toks.subList(0, tokpat.size()));
    //print(structure(tokpat) + " on " + structure(toks) + " => " + structure(m));
    if (m == null)
      return false;
    else {
      if (matches != null) {
        matches.m = new S[m.length+1];
        arraycopy(m, matches.m);
        matches.m[m.length] = join(toks.subList(tokpat.size(), toks.size())); // for Matches.rest()
      }
      return true;
    }
  }