∀ {α : Type u} {β : α → Type v} {x : BEq α} {x_1 : Hashable α} {m : Std.DHashMap α β} [inst : LawfulBEq α]
  {k k' fallback : α} {f : β k → β k},
  (m.modify k f).getKeyD k' fallback = if (k == k') = true then if k ∈ m then k else fallback else m.getKeyD k' fallback