Update chapters/classes.tex

Co-authored-by: Henrik Tidefelt <henrikt@wolfram.com>

chapters/classes.tex

@@ -385,7 +385,7 @@ \subsection{Component Variability Prefixes discrete, parameter, constant}\label{
 \begin{nonnormative}
 For \lstinline!Real! variables we can distinguish two subtly different categories: discrete-time and piecewise constant.
 For a \lstinline!Real! variable, being discrete-time is equivalent to being assigned in a when-clause.
-The discrete-time variables can be used as argument to \lstinline!pre!.
+A variable used as argument to \lstinline!pre! outside a when-clause must be discrete-time.
 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.