∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {β : Type v} {m : Std.ExtDHashMap α fun x => β} [inst : EquivBEq α]
  [inst_1 : LawfulHashable α] {k k' fallback : α} {f : β → β},
  (Std.ExtDHashMap.Const.modify m k f).getKeyD k' fallback =
    if (k == k') = true then if k ∈ m then k else fallback else m.getKeyD k' fallback