 Nonempty computably enumerable sets are primitive recursive ranges

Every nonempty computably enumerable set $X\subseteq \mathbb{N}$ is the range of a primitive recursive function $f:\mathbb{N}\to \mathbb{N}$. Choose $x_0\in X$, decode $n=⟨x,t⟩$ using a primitive recursive pairing function, and output $x$ if the recognizing machine halts within $t$ steps, or $x_0$ otherwise. The bounded halting predicate makes this function primitive recursive, and every member of $X$ occurs for a sufficiently large bound. The empty set is excluded because such a function is total.