Clarify simple function call.

chapters/equations.tex

@@ -60,6 +60,21 @@ \subsection{Simple Equality Equations}\label{simple-equality-equations}
 \end{nonnormative}
 
 
+\subsection{Side-Effect Equations}\label{side-effect-equations}
+
+An equation in the following form calls the given function or operator:
+\begin{itemize}
+\item \lstinline!function_name(inexpr1, inexpr2);!
+\end{itemize}
+It behaves similarly to a simple equation (calling the function or operator), even if it uses a different production in the grammar.
+It is used for special operators like \lstinline!reinit! (\cref{reinit}), \lstinline!assert! (\cref{assert}), and \lstinline!terminate! (\cref{terminate}).
+
+\begin{nonnormative}
+If the function has any outputs they will be ignored.
+That case is not recommended, but it is allowed to avoid having special rules in function-subtyping \cref{function-compatibility-or-function-subtyping-for-functions}.
+\end{nonnormative}
+
+
 \subsection{For-Equations -- Repetitive Equation Structures}\label{for-equations-repetitive-equation-structures}
 
 The syntax of a \lstinline!for!-equation\index{for@\robustinline{for}!equation}\index{loop@\robustinline{loop}!for-equation@\robustinline{for}-equation} is given by \productionref{for-equation} in the grammar.