sbool charArrayRegionMatches(char[] a, int aFrom, char[] b, int bFrom, int n) {
  if (aFrom < 0 || bFrom < 0) false;
  if (aFrom+n > l(a)) false;
  if (bFrom+n > l(b)) false;
  for i to n:
    if (a[aFrom+i] != b[bFrom+i])
      false;
  true;
}