1081.lean:7:2-7:5: error: type mismatch
  rfl
has type
  ?m = ?m
but is expected to have type
  f 0 y = y
1081.lean:23:4-23:9: error: type mismatch
  rfl
has type
  ?m = ?m
but is expected to have type
  insert a ⟨0, ⋯⟩ v = cons a v