red.lean:9:19: error: type mismatch, expression
  rfl
has type
  ?m_2 = ?m_2
but is expected to have type
  f = g
red.lean:12:0: error: "eliminator" elaborator type mismatch, term
  rfl
has type
  ?m_2 = ?m_2
but is expected to have type
  f a = a
Additional information:
red.lean:12:0: context: the inferred motive for the eliminator-like application is
  λ (_x : ℕ), f a = a
red.lean:17:0: error: "eliminator" elaborator type mismatch, term
  rfl
has type
  ?m_2 = ?m_2
but is expected to have type
  f a = a
Additional information:
red.lean:17:0: context: the inferred motive for the eliminator-like application is
  λ (_x : ℕ), f a = a