Just give references for discrete-time requirements in example

chapters/equations.tex

@@ -637,11 +637,9 @@ \section{Events and Synchronization}\label{events-and-synchronization}
 when noEvent(x1 > 1) or x2 > 10 then // When-condition must be discrete-time
   close = true;
 end when;
-above1 = noEvent(x1 > 1);            // above1 is discrete-time variable
+above1 = noEvent(x1 > 1);            // Boolean equation must be discrete-time
 \end{lstlisting}
-The when-condition error is a direct violation of the requirement that the when-condition must be discrete-time.
-The error in the equation with \lstinline!above1! is only indirect; since \lstinline!above1! requires a discrete-time solution, it cannot be solved from this equation.
-Consequently, this equation cannot be solved for any variable, a structural singularity condition that will be reported in tool-specific ways.
+The when-condition rule is stated in \cref{when-equations}, and the rule for a non-\lstinline!Real! equation is stated in \cref{discrete-time-expressions}.
 \end{example}
 
 Modelica is based on the synchronous data flow principle (\cref{synchronous-data-flow-principle-and-single-assignment-rule}).