∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.DTreeMap.Raw α β cmp} [inst : Std.TransCmp cmp]
  [inst_1 : Std.LawfulEqCmp cmp] (h : t.WF) {k k' : α} {f : Option (β k) → Option (β k)} {hc : k' ∈ t.alter k f},
  (t.alter k f).get k' hc = if heq : cmp k k' = Ordering.eq then cast ⋯ ((f (t.get? k)).get ⋯) else t.get k' ⋯