Linear program with $\pm 1$ constraints I am trying to formulate a constraint as follows ($X, Y, Z$ are either $-1$ or $1$): 
If $Z$ and $Y$ both equal $-1$, then $X$ must be $1$. But, if either $Z$ or $Y$ are not $-1$, then $X$ can be $-1$ or $1$.
I came up with $Z\cdot Y \le X$, but that limits $X$ to $1$ if both $Z$ and $Y$ are $1$ ($X$ can be $0$ if both are $1$).
Any advice on a more accurate constraint? 
Thank you.

Let $X=2x-1,Y=2y-1,Z=2z-1$, where $x,y,z\in\{0,1\}$.  Then you want to model $\neg z \land \neg y \implies x$. Rewrite in conjunctive normal form to obtain a linear constraint:
$$\begin{equation}
\neg (\neg z \land \neg y) \lor x \\
z \lor y \lor x \\
z+y+x\ge 1\\
(Z+1)/2+(Y+1)/2+(X+1)/2\ge 1\\
Z+Y+X\ge -1
\end{equation}$$
