Clarify elsewhen (modelica#3217)

* Clarify elsewhen
Closes modelica#3216

* Update chapters/equations.tex

chapters/equations.tex

@@ -398,16 +398,33 @@ \subsubsection{Single Assignment Rule Applied to When-Equations}\label{applicati
 \begin{lstlisting}[language=modelica]
 model WhenPriority
   Boolean close;   // Correct model: close defined by two equations!
-algorithm
+equation
   $\ldots$
   when condition1 then
-    close := true;
+    close = true;
   elsewhen condition2 then
-    close := false;
+    close = false;
   end when;
   $\ldots$
 end WhenPriority;
 \end{lstlisting}
+An alternative to \lstinline!elsewhen! (in an equation or algorithm) is to use an algorithm with multiple \lstinline!when!-statements.
+However, both statements will be executed if both conditions become \lstinline!true! at the same time.
+Therefore they must be in reverse order to preserve the priority, and any side-effect would require more care.
+\begin{lstlisting}[language=modelica]
+model WhenPriorityAlg
+  Boolean close;   // Correct model: close defined by two when-statements!
+algoritmh
+  $\ldots$
+  when condition2 then
+    close := false;
+  end when;
+  when condition1 then
+    close := true;
+  end when;
+  $\ldots$
+end WhenPriorityAlg;
+\end{lstlisting}
 \end{nonnormative}
 \end{itemize}