sub matchrule {
  my ( $old, $highs, $word ) = @_;
  $old eq $word and return 0;
  my $newmask = ( $old ^ $word ) =~ tr/\0/\xff/cr;
  ( $newmask & $highs ) =~ tr/5// and return 0;
  my $tiles = "@tiles";
  $tiles =~ s/$_// or return 0 for ( $newmask & $word ) =~ /\w/g;
  return 1;
}