A set operator forms the  union,  intersection,  difference,  or
  exclusive-OR of two sets, compares two sets, or tests an ordinal
  value for inclusion in a set.  Its result is either a set  or  a
  Boolean value.

  Set Operators:

       operator  |  example  |    result
       --------------------------------------------
          +         A + B      Set that is the union of sets A and B
          *         A * B      Set that is the intersection of sets A
                               and B
          -         A - B      Set of those elements in set A that are
                               not also in set B
          <>        A <> B     TRUE if set A is not equal to set B
          <=        A <= B     TRUE if set A is a subset of set B
          >=        A >= B     TRUE if set B is a subset of set A
          IN        C IN B     TRUE if C is an element of set B
          NOT IN    C NOT IN B TRUE if C is not an element of B