I am very new to coq so if you only say intros. I don't know what to introduce. So being specific like (Ex. intros p q.) would be very helpful.
Fortunately, your goal can be automatically solved by the auto
tactic. If you use its variant info_auto
, Coq tells you which steps lead to the solution.
Goal forall P Q : Prop, ((((P -> Q) -> P) -> P) -> Q) -> Q.
info_auto.
Then, you may replay the proof by yourself, and add comments and explicit variables names to the intros
tactics, and you will understand the strategy used by auto
(what to do when the conclusion is a forall
or an implication, or an atomic proposition).
Restart.
intros P Q H.
apply H (* no-choice ! *).
intro H0.
apply H0. (* no other possibility ! *)
intro p.
apply H. (* no alternative ! *)
intros _.
assumption.
Qed.
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