là nhà điều hành giao nhau, đôi khi được gọi là nắp. Nhà điều hành công đoàn có hai bộ và các hình thức một tập hợp có chứa tất cả các yếu tố trong thiết lập với bản sao loại bỏ. Như vậy, kết quả của biểu thức {File1, file2, thuế, biên dịch} là tập hợp {Filel, file2, thuế, biên dịch, NewTax, D2, D3} | CHAPTER 25 FORMAL METHODS 685 The operator c is similar to c. However if its operands are equal it has the value true. Thus the value of the predicate HD1 LP4 RC5 c HD1 RC2 HD3 LP1 LP4 LP6 is false and the predicate HD1 LP4 RC5 c HD1 LP4 RC5 is true. A special set is the empty set 0. This corresponds to zero in normal mathematics. The empty set has the property that it is a subset of every other set. Two useful identities involving the empty set are 0 u A A and 0 n A 0 for any set A where u is known as the union operator sometimes known as cup n is the intersection operator sometimes known as cap. _Juote The union operator takes two sets and forms a set that contains all the elements Mathematical structures are among the most beautiful discoveries made by in the set with duplicates eliminated. Thus the result of the expression File1 File2 Tax Compiler u NewTax D2 D3 File2 is the set the human mind. Douglas Hofstadter Filel File2 Tax Compiler NewTax D2 D3 The intersection operator takes two sets and forms a set consisting of the common elements in each set. Thus the expression 12 4 99 1 n 1 13 12 77 results in the set 12 1 . The set difference operator as the name suggests forms a set by removing the elements of its second operand from the elements of its first operand. Thus the value of the expression New Old TaxFile Sysparam Old SysParam results in the set New TaxFile . The value of the expression a b c d n x y will be the empty set 0. The operator always delivers a set however in this case there are no common elements between its operands so the resulting set will have no elements. The final operator is the cross product X sometimes known as the Cartesian product. This has two operands which are sets of pairs. The result is a set of pairs where each pair consists of an element taken from the first operand combined with an 686 PART FIVE ADVANCED TOPICS IN SOFTWARE ENGINEERING element from the second operand. An example of an expression involving the cross product
