Actual definition of discrete-time variable.

chapters/classes.tex

@@ -344,13 +344,14 @@ \subsection{Component Variability Prefixes discrete, parameter, constant}\label{
 \item
   A variable \lstinline!vc! declared with the \lstinline!parameter!\indexinline{parameter} prefix remains constant during transient analysis, with a value determined by the initialization problem.
 \item
-  A \emph{discrete-time} variable \lstinline!vd! has a vanishing time derivative between events.
+  A \emph{discrete-time} variable \lstinline!vd! is a variable declared with the \lstinline!discrete!\indexinline{discrete} prefix, or a variable without any prefix that is not a continuous-time variable.
+  It has a vanishing time derivative between events.
   Note that this is not the same as saying that \lstinline!der(vd)=0! almost everywhere,
   as the derivative is not even defined at the events, and it is not legal
-  to apply \lstinline!der! to discrete-time variables as they are not continuous. During transient analysis the variable
-  can only change its value at event
-  instants (see \cref{events-and-synchronization}).
+  to apply \lstinline!der! to discrete-time variables as they are not continuous. 
+  During transient analysis the variable can only change its value at event instants (see \cref{events-and-synchronization}).
 \item
+  A continuous-time variable is a \lstinline!Real! variable without any prefix that is not assigned in a \lstinline!when!-clause.
   A \emph{continuous-time} variable \lstinline!vn! may have a non-vanishing time
   derivative (\lstinline!der(vn)<>0! possible) and may also
   change its value discontinuously at any time during transient analysis