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

chapters/classes.tex

@@ -353,10 +353,10 @@ \subsection{Component Variability Prefixes discrete, parameter, constant}\label{
   It is not allowed to apply \lstinline!der! to discrete-time variables.
   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 \firstuse{continuous-time variable}\index{continuous-time!variable}\index{component variability!continuous-time} 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 (provided \lstinline!der(vn)! is allowed this can be expressed as \lstinline!der(vn) <> 0!) 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.
-  Note that \lstinline!der(vn)! is not allowed if there are any discontinuities, see \cref{modelica:der}.
+  Regarding existence of \lstinline!der(vn)!, see \cref{modelica:der}.
 \end{itemize}
 
 Components declared as \lstinline!constant! shall have an associated declaration equation with a constant expression, if the constant is directly in the simulation model, or used in the simulation model.