static S[] subStringArray(S[] b, int start, int end) {
  start = max(start, 0); end = min(end, l(b));
  if (start == 0 && end == l(b)) ret b;
  if (start >= end) ret new S[0];
  S[] x = new S[end-start];
  System.arraycopy(b, start, x, 0, end-start);
  ret x;
}