csimpAttr.lean:7:2-7:7: error: invalid 'csimp' theorem, only constant replacement theorems (e.g., `@f = @g`) are currently supported.

[init]
def f (x_1 : obj) : obj :=
  let x_2 : obj := Nat.add x_1 x_1;
  let x_3 : obj := Nat.add x_2 x_2;
  ret x_3