sub crep {
  my ($str, $chr, $rep, $nth) = @_;
  my $pos = 0;
  while (--$nth > 0) {
    $pos = index $str, $chr, $pos;
    last if $pos < 0;
    $pos++;
  }
  substr ($str, $pos) =~ s/$chr/$rep/g if $pos >= 0;
  return $str;
}