my $newmask = ( $old ^ $word ) =~ tr/\0/\xff/cr;
    $flip and ( $board, $heights ) = flip $board, $heights;
    substr $board, $pos, length $word, $word;

    #say "new mask is $newmask";
    substr $heights, $pos, length $highs,
      ( $highs & $newmask ) =~ tr/0-4/1-5/r | ( $highs & ~$newmask );
    $flip and ( $board, $heights ) = flip $board, $heights;
    my $tiles = join '', @tiles;
    say "word is $word";
    $tiles =~ s/$_// for split //, $word & $newmask;