Again let Σ be an alphabet. Let "A" be the set of all regular languages over Σ (or the set of all context-free languages over Σ; or the set of all recursive languages over Σ; or the set of "all" languages over Σ). Then the union (written as +) and the concatenation (written as ·) of two elements of "A" again belong to "A", and so does the Kleene star operation applied to any element of "A". We obtain a Kleene algebra "A" with 0 being the empty set and 1 being the set that only contains the empty string.