(SymmetricDifference=
  A B x symdiff
.   !arg:(?A.?B)
  & :?symdiff
  & (   !A !B
      :   ?
          ( %@?x
          & ( !A:? !x ?&!B:? !x ?
            | !symdiff:? !x ?
            | !symdiff !x:?symdiff
            )
          & ~
          )
          ?
    | !symdiff
    ));