sub common {
   if ( $_[0] eq $_[1] ) {
         return( $_[0], "", "" );
   } else {
      use bytes;
      my ( $pos, $len0, $len1 ) = ( 0, length($_[0]), length($_[1]) );
      # find the offset of the first byte that differs
      while ( vec($_[0], $pos, 8) == vec($_[1], $pos, 8) ) {
         $pos++;
         last unless ( $pos < $len0 && $pos < $len1 );
      }
      return(
         unpack("a${pos}", $_[0]),
         unpack("x${pos}a*", $_[0]),
         unpack("x${pos}a*", $_[1]),
      );
   }
}