Check whether the following can be probability functions for discrete
random variables:
\begin{cases}
2/3, & x = -2 \\
1/3, & x = 5 \\
0, & \text{elsewhere.}
\end{cases}
\begin{cases}
3/4, & x = -3 \\
2/4, & x = 0, \\
-1/4, & x = 2 \\
0, & \text{elsewhere.}
\end{cases}
\begin{cases}
3/5, & x = 0 \\
3/5, & x = 1 \\
0, & \text{elsewhere.}
\end{cases}
