// returns from C to C
static S jextract(S pat, S s) {
  L<S> tok = javaTok(s);
  L<S> tokpat = javaTok(pat);
  jfind_preprocess(tokpat);
  int i = jfind(tok, tokpat);
  if (i < 0) null;
  int j = i + l(tokpat) - 2;
  ret join(subList(tok, i, j));
}