let_fun f := fun x => x * 2;
let_fun x := 1;
let_fun y := x + 1;
f (y + x) : Nat
a b : Nat
h1 : a = 0
h2 : b = 0
⊢ b <
    let_fun x := 1;
    x + x
(let_fun this := id;
  this)
  1 : Nat
a b : Nat
h : a > b
⊢ b < a
let_fun n := 5;
⟨[], ⋯⟩ : { as // as.length ≤ 5 }
rfl : (let_fun n := 5;
  n) =
  let_fun n := 5;
  n
rfl : 2 = 2
5
let_fun n := 5;
n