Update chapters/classes.tex

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

chapters/classes.tex

@@ -354,7 +354,7 @@ \subsection{Component Variability Prefixes discrete, parameter, constant}\label{
   During transient analysis the variable can only change its value at event instants (see \cref{events-and-synchronization}).
 \item
   A \emph{continuous-time}\index{continuous-time!variable}\index{component variability!continuous-time} variable is a \lstinline!Real! variable without any prefix that is not assigned in a \lstinline!when!-clause.
-  A continuous-time variable \lstinline!vn! may have a non-vanishing time derivative (\lstinline!der(vn)<>0! possible - if \lstinline!der(vn)! is allowed) and may also change its value discontinuously at any time during transient analysis (see \cref{events-and-synchronization}).
+  A continuous-time variable \lstinline!vn! may have a non-vanishing time derivative (\lstinline!der(vn) <> 0! is possible, provided \lstinline!der(vn)! is defined) and may also change its value discontinuously at any time during transient analysis (see \cref{events-and-synchronization}).
   It may also contain a combination of these effects.
   If there are any discontinuities the variable is not differentiable.
 \end{itemize}