On page 100, A Mathematical Introduction to Logic, Herbert B. Enderton(2ed),
Assume that the language has equality and a two-place predicate symbol $P$. For each of the following conditions, find a sentence $σ$ such that the structure $\mathfrak A$ is a model of $σ$ iff the condition is met. (a) $|\mathfrak A|$ has exactly two members.
My first attempt is the sentence $\forall x \exists y,z((y \neq x \land z \neq x) \to y = z)$, or equivalently, $\forall x \exists y,z(y \neq x \to (z \neq x \to y = z))$.
It seems to me this is incorrect, because if the universe is a singleton, then there doesn't exist $y$ and $z$ that don't equal $x$, which implies that the sentence is vacuously true, which lead to my second version:
$\forall x \exists y,z((y \neq x \land z \neq x)\land((y \neq x \land z \neq x) \to y = z))$ which is $\forall x \exists y,z(y \neq x \land z \neq x \land y = z)$. This is also incorrect, since it doesn't exclude the scenario that the universe contains more than elements.
Thus, finally, should the sentence look like $\forall x ((\exists y,z((y \neq x \land z \neq x) \to y = z))\land(\exists y,z(y \neq x \land z \neq x)))$ ? Is there any alternative that is less cumbersome?