subst_bug.lean:4:2: error:invalid 'subst' tactic, 'a' occurs in the other side of the equation
proof state:
f : nat → nat,
a b : nat,
h₁ : f a = a
⊢ f (f a) = a
subst_bug.lean:5:0: error: don't know how to synthesize placeholder
f : nat → nat,
a b : nat
⊢ f a = a → f (f a) = a
subst_bug.lean:5:0: error: failed to add declaration 'example' to environment, value has metavariables
remark: set 'formatter.hide_full_terms' to false to see the complete term
  ?M_1