
  -spec symmetric_partition(Set1, Set2) -> {Set3, Set4, Set5}
                               when
                                   Set1 :: a_set(),
                                   Set2 :: a_set(),
                                   Set3 :: a_set(),
                                   Set4 :: a_set(),
                                   Set5 :: a_set().

  Returns a triple of sets:

   • Set3 contains the elements of Set1 that do not belong to 
     Set2.

   • Set4 contains the elements of Set1 that belong to Set2.

   • Set5 contains the elements of Set2 that do not belong to 
     Set1.
