-- Lean 4 proof step from Mathlib.Computability.PartrecCode (goal)
Goal:
x : Unit
p n : ℕ
this : List.range p = List.range (Nat.pair (unpair p).1 (encode (ofNat Code (unpair p).2)))
k : ℕ
c : Code
nk : n ∈ List.range k
⊢ Nat.rec (motive := fun (x : ℕ) ↦ Option ℕ) Option.none
      (fun (n_1 : ℕ) (n_ih : Option ℕ) ↦
        rec (motive := fun (x : Code) ↦ Option ℕ) (some 0) (some n.succ) (some (unpair n).1) (some (unpair n).2)
          (fun (cf cg : Code) (x x_1 : Option ℕ) ↦ do
            let x ←
              lup
                  (List.map
                    (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                    (List.range (Nat.pair k (encode c))))
                  (k, cf) n
            let y ←
              lup
                  (List.map
                    (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                    (List.range (Nat.pair k (encode c))))
                  (k, cg) n
            some (Nat.pair x y))
          (fun (cf cg : Code) (x x_1 : Option ℕ) ↦ do
            let x ←
              lup
                  (List.map
                    (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                    (List.range (Nat.pair k (encode c))))
                  (k, cg) n
            lup
                (List.map
                  (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                  (List.range (Nat.pair k (encode c))))
                (k, cf) x)
          (fun (cf cg : Code) (x x_1 : Option ℕ) ↦
            Nat.rec (motive := fun (x : ℕ) ↦ Option ℕ)
              (lup
                (List.map
                  (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                  (List.range (Nat.pair k (encode c))))
                (k, cf) (unpair n).1)
              (fun (n_2 : ℕ) (n_ih : Option ℕ) ↦ do
                let i ←
                  lup
                      (List.map
                        (fun (n : ℕ) ↦
                          List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                        (List.range (Nat.pair k (encode c))))
                      (n_1, c) (Nat.pair (unpair n).1 n_2)
                lup
                    (List.map
                      (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                      (List.range (Nat.pair k (encode c))))
                    (k, cg) (Nat.pair (unpair n).1 (Nat.pair n_2 i)))
              (unpair n).2)
          (fun (cf : Code) (x : Option ℕ) ↦ do
            let x ←
              lup
                  (List.map
                    (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                    (List.range (Nat.pair k (encode c))))
                  (k, cf) n
            Nat.rec (motive := fun (x : ℕ) ↦ Option ℕ) (some (unpair n).2)
                (fun (n_2 : ℕ) (n_ih : Option ℕ) ↦
                  lup
                    (List.map
                      (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                      (List.range (Nat.pair k (encode c))))
                    (n_1, c) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
                x)
          c)
      k =
    evaln k c n
have zero : ∀ (nk : n ∈ List.range 0),
  Nat.rec (motive := fun (x : ℕ) ↦ Option ℕ) Option.none
      (fun (n_1 : ℕ) (n_ih : Option ℕ) ↦
        rec (motive := fun (x : Code) ↦ Option ℕ) (some 0) (some n.succ) (some (unpair n).1) (some (unpair n).2)
          (fun (cf cg : Code) (x x_1 : Option ℕ) ↦ do
            let x ←
              lup
                  (List.map
                    (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                    (List.range (Nat.pair 0 (encode c))))
                  (0, cf) n
            let y ←
              lup
                  (List.map
                    (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                    (List.range (Nat.pair 0 (encode c))))
                  (0, cg) n
            some (Nat.pair x y))
          (fun (cf cg : Code) (x x_1 : Option ℕ) ↦ do
            let x ←
              lup
                  (List.map
                    (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                    (List.range (Nat.pair 0 (encode c))))
                  (0, cg) n
            lup
                (List.map
                  (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                  (List.range (Nat.pair 0 (encode c))))
                (0, cf) x)
          (fun (cf cg : Code) (x x_1 : Option ℕ) ↦
            Nat.rec (motive := fun (x : ℕ) ↦ Option ℕ)
              (lup
                (List.map
                  (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                  (List.range (Nat.pair 0 (encode c))))
                (0, cf) (unpair n).1)
              (fun (n_2 : ℕ) (n_ih : Option ℕ) ↦ do
                let i ←
                  lup
                      (List.map
                        (fun (n : ℕ) ↦
                          List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                        (List.range (Nat.pair 0 (encode c))))
                      (n_1, c) (Nat.pair (unpair n).1 n_2)
                lup
                    (List.map
                      (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                      (List.range (Nat.pair 0 (encode c))))
                    (0, cg) (Nat.pair (unpair n).1 (Nat.pair n_2 i)))
              (unpair n).2)
          (fun (cf : Code) (x : Option ℕ) ↦ do
            let x ←
              lup
                  (List.map
                    (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                    (List.range (Nat.pair 0 (encode c))))
                  (0, cf) n
            Nat.rec (motive := fun (x : ℕ) ↦ Option ℕ) (some (unpair n).2)
                (fun (n_2 : ℕ) (n_ih : Option ℕ) ↦
                  lup
                    (List.map
                      (fun (n : ℕ) ↦ List.map (evaln (unpair n).1 (ofNat Code (unpair n).2)) (List.range (unpair n).1))
                      (List.range (Nat.pair 0 (encode c))))
                    (n_1, c) (Nat.pair (unpair n).1 ((unpair n).2 + 1)))
                x)
          c)
      0 =
    evaln 0 c n
Tactic: rcases k with - | k'
Cleared hypotheses: k, nk