 possibly with repetitions—e.g., the constant function \(\const_{17}(x)\) enumerates the singleton set \(\{17\}\), which is thereby c.e. It is easy to see that a computable set \(A\) is computably enumerable. For if \(A = \emptyset\), then \(A\) is c.e. by definition. And if \(A \neq \emptyset\), we may choose \(a \in A\) and then define 