Add variability rule for multiple return assignment

chapters/operatorsandexpressions.tex

@@ -1347,6 +1347,8 @@ \section{Variability of Expressions}\label{variability-of-expressions}
 \begin{itemize}
 \item
   For an assignment \lstinline!v := expr! or binding equation \lstinline!v = expr!, \lstinline!v! must be declared to be at least as variable as \lstinline!expr!.
+\item
+  For multiple return assignment \lstinline!(x1, $\ldots$, xn) := expr! (see \cref{assignments-from-called-functions-with-multiple-results}), all of \lstinline!x1!, \ldots, \lstinline!xn! must be declared to be at least as variable as \lstinline!expr!.
 \item
   When determining whether an equation can contribute to solving for a variable \lstinline!v! (for instance, when applying the perfect matching rule, see \cref{synchronous-data-flow-principle-and-single-assignment-rule}), the equation can only be considered contributing if the resulting solution would be at most as variable as \lstinline!v!.
 \item