simp only [this, children, Approx.node, 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
this : i = cast (_ : B C (destruct x).fst = B C (Approx.node (approx x (Nat.succ n)))) j
⊢ Approx.children (approx x (Nat.succ n)) i = approx (PSigma.snd (destruct x) j) n
no goals