rw [node_thm]
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
⊢ B C (destruct x).fst = B C (Approx.node (approx x (Nat.succ 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
⊢ B C (destruct x).fst = B C (node x)