On the existential quantifier

Section on_ex. 
  Variables 
   (A:Type)
   (P Q:A -> Prop).

 Lemma ex_or : (exists x:A, P x \/ Q x) -> ex P \/ ex Q.
 
 Lemma ex_or_R : ex P \/ ex Q -> (exists x:A, P x \/ Q x).

 Lemma two_is_three : (exists x:A, forall R : A->Prop, R x) -> 2 = 3.

 Lemma forall_no_ex : (forall x:A, P x) -> ~(exists y:A, ~ P y).

End on_ex.

Solution