sub leftover {
  my ($full, $part) = @_;
  my @count;
  $count[ord $1]++ while $full =~ /(\w)/g;
  while ($part =~ /(\w)/g) {
    return undef if --$count[ord $1] < 0;
  }
  my $return;
  $count[$_] and $return .= chr($_) x $count[$_] for 0..$#count;
  $return;
}