intro x n i j heq
C : Container
⊢ ∀ (x : M C) (n : Nat) (i : B C (Approx.node (approx x (Nat.succ n)))) (j : B C (destruct x).fst),
    HEq i j → Approx.children (approx x (Nat.succ n)) i = approx (PSigma.snd (destruct x) j) n
C : Container
x : M C
n : Nat
i : B C (Approx.node (approx x (Nat.succ n)))
j : B C (destruct x).fst
heq : HEq i j
⊢ Approx.children (approx x (Nat.succ n)) i = approx (PSigma.snd (destruct x) j) n