DEFINITION ex2_sym()
TYPE =
∀A:Set
.∀P:A→Prop
.∀Q:A→Prop.(ex2 A λx:A.P x λx:A.Q x)→(ex2 A λx:A.Q x λx:A.P x)
BODY =
assume A: Set
assume P: A→Prop
assume Q: A→Prop
suppose H: ex2 A λx:A.P x λx:A.Q x
we proceed by induction on H to prove ex2 A λx:A.Q x λx:A.P x
case ex_intro2 : x:A H0:P x H1:Q x ⇒
the thesis becomes ex2 A λx0:A.Q x0 λx0:A.P x0
by (ex_intro2 . . . . H1 H0)
ex2 A λx0:A.Q x0 λx0:A.P x0
we proved ex2 A λx:A.Q x λx:A.P x
we proved
∀A:Set
.∀P:A→Prop
.∀Q:A→Prop.(ex2 A λx:A.P x λx:A.Q x)→(ex2 A λx:A.Q x λx:A.P x)