How do I pick a single element from a set that satisfies a property? For instance, I want to write something like this:
$$S = S - \{s \in S \ | \ s \text{ is pretty}\}$$
But with $\{s \in S \ | \ s \text{ is pretty}\}$ I want to pick a single element (any of the pretty ones).
Edit: If possible, the answer should be in a syntax similar to the definition of a set.