Add circuit constraint section on conditional equality.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

protocol/protocol.tex

@@ -9527,6 +9527,16 @@ A boolean constraint $b \in \bit$ can be implemented as:
 \end{formulae}
 
 
+\introlist
+\subsubsubsection{Conditional equality} \label{cctcondeq}
+
+The constraint ``either $a = 0$ or $b = c$'' can be implemented as:
+
+\begin{formulae}
+  \item $\constraint{a}{b - c}{0}$
+\end{formulae}
+
+
 \introlist
 \subsubsubsection{Selection constraints} \label{cctselection}