Let x and y be strings and let l be any language. we say that x and y are distinguishable by l if some string z exists whereby exactly one of the strings xz and yz is a member of l; otherwise, for every stringz, we havexz ∈ l whenever yz ∈ l and we say that x and y are indistinguishable by l. ifx and y are indistinguishable by l, we writex ≡l y. show that≡l is an equivalence relation.