Theorem elprg

Description: A member of an unordered pair of classes is one or the other of them. Exercise 1 of TakeutiZaring p. 15, generalized. (Contributed by NM, 13-Sep-1995)

	Ref	Expression
Assertion	elprg	⊢ ( 𝐴 ∈ 𝑉 → ( 𝐴 ∈ { 𝐵 , 𝐶 } ↔ ( 𝐴 = 𝐵 ∨ 𝐴 = 𝐶 ) ) )

Proof

Step	Hyp	Ref	Expression
1		eqeq1	⊢ ( 𝑥 = 𝐴 → ( 𝑥 = 𝐵 ↔ 𝐴 = 𝐵 ) )
2		eqeq1	⊢ ( 𝑥 = 𝐴 → ( 𝑥 = 𝐶 ↔ 𝐴 = 𝐶 ) )
3	1 2	orbi12d	⊢ ( 𝑥 = 𝐴 → ( ( 𝑥 = 𝐵 ∨ 𝑥 = 𝐶 ) ↔ ( 𝐴 = 𝐵 ∨ 𝐴 = 𝐶 ) ) )
4		dfpr2	⊢ { 𝐵 , 𝐶 } = { 𝑥 ∣ ( 𝑥 = 𝐵 ∨ 𝑥 = 𝐶 ) }
5	3 4	elab2g	⊢ ( 𝐴 ∈ 𝑉 → ( 𝐴 ∈ { 𝐵 , 𝐶 } ↔ ( 𝐴 = 𝐵 ∨ 𝐴 = 𝐶 ) ) )