sbool jmatchStart(S pat, S s) {
  ret jmatchStart(pat, s, null);
}

// matches are as you expect, plus an extra item for the rest string
sbool jmatchStart(S pat, S s, Matches matches) {
  if (s == null) false;
  LS tokpat = javaTok(pat), toks = javaTok(s);
  ret jmatchStart(tokpat, toks, matches);
}
  
sbool jmatchStart(S pat, LS toks, Matches matches default null) {
  if (toks == null) false;
  LS tokpat = javaTok(pat);
  ret jmatchStart(tokpat, toks, matches);
}

sbool jmatchStart(LS tokpat, LS toks, Matches matches) {
  if (tokpat == null || toks == null || 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;
  }
}

Began life as a copy of #1001793