rewrite [Nat.succ_sub_one, Nat.mul_div_right _ (by decide)] at this
case node_left
i✝ j i : Nat
h : Descendant (2 * i + 1) j
a✝ : 2 * i + 1 < j → Descendant (2 * i + 1) ((j - 1) / 2)
hij : i < j
this : Descendant ((2 * i + 1 - 1) / 2) ((j - 1) / 2)
⊢ Descendant i ((j - 1) / 2)
case node_left
i✝ j i : Nat
h : Descendant (2 * i + 1) j
a✝ : 2 * i + 1 < j → Descendant (2 * i + 1) ((j - 1) / 2)
hij : i < j
this : Descendant i ((j - 1) / 2)
⊢ Descendant i ((j - 1) / 2)