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

static bool matchX(S pat, S s, Matches m) {
  if (endsWith(pat, "...")) ret matchStartX(pat, s, m);
  if (startsWith(pat, "...")) ret matchEndX(pat, s, m);
  ret match(pat, s, m);
}