sub usingtr_d {
    my $a = $aa;
    my $b = $bb;
    # Delete ACGT first, assuming these are the
    # most common characters.
    abs($a =~ y/A//d - $b =~ y/A//d) +
    abs($a =~ y/C//d - $b =~ y/C//d) +
    abs($a =~ y/G//d - $b =~ y/G//d) +
    abs($a =~ y/T//d - $b =~ y/T//d) +
    abs($a =~ y/B// - $b =~ y/B//) +
    abs($a =~ y/D// - $b =~ y/D//) +
    abs($a =~ y/E// - $b =~ y/E//) +
    abs($a =~ y/F// - $b =~ y/F//) +
    abs($a =~ y/H// - $b =~ y/H//) +
    abs($a =~ y/I// - $b =~ y/I//) +
    abs($a =~ y/J// - $b =~ y/J//) +
    abs($a =~ y/K// - $b =~ y/K//) +
    abs($a =~ y/L// - $b =~ y/L//) +
    abs($a =~ y/M// - $b =~ y/M//) +
    abs($a =~ y/N// - $b =~ y/N//) +
    abs($a =~ y/O// - $b =~ y/O//) +
    abs($a =~ y/P// - $b =~ y/P//) +
    abs($a =~ y/Q// - $b =~ y/Q//) +
    abs($a =~ y/R// - $b =~ y/R//) +
    abs($a =~ y/S// - $b =~ y/S//) +
    abs($a =~ y/U// - $b =~ y/U//) +
    abs($a =~ y/V// - $b =~ y/V//) +
    abs($a =~ y/W// - $b =~ y/W//) +
    abs($a =~ y/X// - $b =~ y/X//) +
    abs($a =~ y/Y// - $b =~ y/Y//) +
    abs($a =~ y/Z// - $b =~ y/Z//);
}