Reformulate the differentiability part.

Using allowed instead of defined, as tools should check that requirement.
Clarifying that der(vn)<>0 is just putting formula behind the text.

chapters/classes.tex

@@ -354,9 +354,9 @@ \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! 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}).
+  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.
-  If there are any discontinuities the variable is not differentiable.
+  Note that \lstinline!der(vn)! is not allowed if there are any discontinuities, 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.