Let $\displaystyle \phi (v)$ be a formula of the languages of set theory. Let X be a set. Then the following is a set: $\displaystyle \{a \in X | \phi [v] \}$, ie all the elements of X with the property $\displaystyle \phi$ form a set.

Write down a formula $\displaystyle \phi (v)$ saying that the set v has exactly one element.

I'm actually stuck here for very long. Is there really an abstract formula that represents all the concrete examples that the set has exactly one element?