You have a typo in the lemma statement, f is applied twice instead of three times.
In Isabelle/HOL equivalent to the above but with a different notation
lemma "∀(f :: bool ⇒ bool). f (f (f b)) = f b" by smt
And with another notation
lemma fixes f :: "bool ⇒ bool" shows "f (f (f b)) = f b" by smt
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You have a typo in the lemma statement, f is applied twice instead of three times.
In Isabelle/HOL equivalent to the above but with a different notation
And with another notation