# are all letters of X in Y?
sub inclusive {
  my ($src, $dst) = @_;
  my %p;
  # I don't know which of these is optimal...
  # so I leave it up to you
  # for my $c (split //, $src) {
  while (defined(my $c = chop $src)) {
    return if !($p{$c} = 1 + index($dst, $c, $p{$c} || 0))
  }
  return 1;
}