calc _ ≤ _ := Nat.mul_le_mul_right _ h8
     _ ≤ _ := h6
x b : ℕ
hb : 2 ≤ b
xpos : 0 < x
h1 : b.digits x ≠ []
h3 :
  (List.mapIdx (fun i a => a * b ^ i) (b.digits x)).dropLast.sum +
      (List.mapIdx (fun i a => a * b ^ i) (b.digits x)).getLast ⋯ =
    x
h4 : List.mapIdx (fun i a => a * b ^ i) (b.digits x) ≠ []
h6 : b ^ (b.digits x).dropLast.length * (b.digits x).getLast h1 ≤ x
h8 : (b.digits x).dropLast.prod ≤ b ^ (b.digits x).dropLast.length
⊢ (b.digits x).dropLast.prod * (b.digits x).getLast h1 ≤ x
no goals