Adding example, and trying to clarify based on review.

chapters/classes.tex

@@ -383,9 +383,27 @@ \subsection{Component Variability Prefixes discrete, parameter, constant}\label{
 definition equations is given in \cref{variability-of-expressions}.
 
 \begin{nonnormative}
-A discrete-time variable is a piecewise constant signal which changes its values only at event instants during simulation.
-There may be additional \lstinline!Real! variables that are piecewise constant without being declared as \lstinline!discrete!.
-Those additional variables may not be used where a \lstininline!discrete! variable is required, but tools may optimize code to only compute them at events.
+For \lstinline!Real! variables we can distinguish two subtly different categories: discrete-time and piecewise constant.
+The discrete-time \lstinline!Real! variables must be assigned in a when-clause.
+The discrete-time variables can be used as argument to \lstinline!pre!.
+The discrete-time variables are a subset of all variables that are piecewise constant and only change at events.
+Tools may optimize code to only compute piecewise constant variables at events.
+
+\begin{lstlisting}[language=modelica]
+model ShowDiscrete
+  discrete Real xd1 "Must be assigned in a when-clause, discrete-time";
+  Real xd2 "Assigned in a when clause (below) and thus discrete-time";
+  Real xc3 "Not discrete-time, but piece-wise constant";
+equation
+  when sample(1,1) then
+    xd1 = pre(xd1) + 1;
+    xd2 = pre(xd2) + 1;
+  end when;
+  // It is legal to use pre for discrete variable outside of when
+  xc3 = xd1 + pre(xd2);
+  // But pre(xc3) would not be legal
+end ShowDiscrete;
+\end{lstlisting}
 
 A \lstinline!parameter! variable is constant during simulation.  This prefix gives the library designer the possibility to express that the physical equations in a library are only valid if some of the used components are constant during simulation.  The same also holds for discrete-time and constant variables.  Additionally, the \lstinline!parameter! prefix allows a convenient graphical user interface in an experiment environment, to support quick changes of the most important constants of a compiled model.  In combination with an if-clause, a \lstinline!parameter! prefix allows removing parts of a model before the symbolic processing of a model takes place in order to avoid variable causalities in the model (similar to \lstinline!#ifdef! in C).  Class parameters can be sometimes used as an alternative.