NAME: -

        FUNCTION: SET difference operator

        OPERATION: Union

        OPERAND TYPE:  Compatible set types

        NOTES:  The result of these operations must conform to the
                rules of set logic:

          An ordinal value c is in a + b only if c is in a or b.
          An ordinal value c is in a - b only if c is in a and not in b.
          An ordinal value c is in a * b only if c is in both a and b.