static bool matchStart(S pat, S s) {
  ret matchStart(pat, s, null);
}

// matches are as you expect, plus an extra item for the rest string
static bool matchStart(S pat, S s, Matches matches) {
  if (s == null) false;
  ret matchStart(pat, parse3_cachedInput(s), matches);
}
  
static bool matchStart(S pat, LS toks, Matches matches) {
  if (toks == null) false;
  L<S> tokpat = parse3_cachedPattern(pat);
  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) false;
  if (matches != null) {
    matches.m = new S[m.length+1];
    arraycopy(m, matches.m);
    matches.m[m.length] = joinSubList(toks, tokpat.size(), toks.size()); // for Matches.rest()
  }
  true;
}