\begin{definition}[Definition:Inverse Cosine/Real]
Let $x \in \R$ be a real number such that $-1 \le x \le 1$.
The '''inverse cosine of $x$''' is the multifunction defined as:
:$\cos^{-1} \left({x}\right) := \left\{{y \in \R: \cos \left({y}\right) = x}\right\}$
where $\cos \left({y}\right)$ is the cosine of $y$.
\end{definition}