// does not return unclosed strings
static L<S> findSubstringsEnclosedBy_inner(S s, char a, char b) {
  int i = 0;
  new L<S> l;
  while ((i = indexOf(s, a, i)) >= 0) {
    int j = indexOf(s, b, i+1);
    if (j < 0) break;
    l.add(substring(s, i+1, j));
    i = j+1;
  }
  ret l;
}