∀ {α : Type u} {m : Std.HashSet.Raw α} [inst : BEq α] [inst_1 : Hashable α] [EquivBEq α] [LawfulHashable α],
  m.WF → ∀ {k a fallback : α}, (m.erase k).getD a fallback = if (k == a) = true then fallback else m.getD a fallback