have : C.B x.destruct.fst = C.B (x.approx (.succ n)).node := by rw [node_thm]; simp [destruct]
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
⊢ i = cast (_ : B C (destruct x).fst = B C (Approx.node (approx x (Nat.succ n)))) j
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
this : B C (destruct x).fst = B C (Approx.node (approx x (Nat.succ n)))
⊢ i = cast (_ : B C (destruct x).fst = B C (Approx.node (approx x (Nat.succ n)))) j