sub rankdwn {
    my ( $yb, $chntot, $incrsdel ) = @_;
    my $cnd_a = $aod[$yb] =~ tr/a/a/;
    my $cnd_y = $aod[$yb] =~ tr/y/y/;

    my $letter = 'a';
    if ( ( $cnd_a + $cnd_y ) <= $incrsdel ) {
        $aod[$yb] =~ s/a/y/gsxm;
        $cnd_y += $cnd_a;
        $letter = 'c';
    }
    while ( $cnd_y < $incrsdel ) {
        my $xda = int rand( $chntot + 1 );
        if ( substr( $aod[$yb], $xda, 1 ) eq $letter ) {
            substr $aod[$yb], $xda, 1, 'y';
            $cnd_y++;
        }
    }
    return;
}