Here's my solution in Coq:
Lemma lemma : forall (f : bool -> bool) (b : bool), f (f (f b)) = f b. intros;case b eqn:G,(f b) eqn:H,(f (f b)) eqn:I;rewrite G,H,I in*;intuition. Qed.
The second line has 80 characters.
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Here's my solution in Coq:
The second line has 80 characters.