my $mask = $keyset1 ^ $keyset2;
                next unless 1+ index $mask, chr( 0 );
                printf "%3d : %3d : ", $key1, $key2;
                print join ', ', map{
                    substr( $mask, $_, 1 ) eq chr(0)  ? $_+1 : ()
                } 0 .. length( $mask )-1;