∀ {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] {a : α} {fallback : β},
  m.contains a = true → Std.ExtDHashMap.Const.get? m a = some (Std.ExtDHashMap.Const.getD m a fallback)