Clarify discrete and remove odd statement about Pantelides.

chapters/classes.tex

@@ -383,14 +383,9 @@ \subsection{Component Variability Prefixes discrete, parameter, constant}\label{
 definition equations is given in \cref{variability-of-expressions}.
 
 \begin{nonnormative}
-A discrete-time variable is a piecewise constant signal which
-changes its values only at event instants during simulation. Such types
-of variables are needed in order that special algorithms, such as the
-algorithm of Pantelides for index reduction, can be applied (it must be
-known that the time derivative of these variables is identical to zero).
-Furthermore, memory requirements can be reduced in the simulation
-environment, if it is known that a component can only change at event
-instants.
+A discrete-time variable is a piecewise constant signal which changes its values only at event instants during simulation.
+There may be additional \lstinline!Real! variables that are piecewise constant without being declared as \lstinline!discrete!.
+Those additional variables may not be used where a \lstininline!discrete! variable is required, but tools may optimize code to only compute them at events.
 
 A \lstinline!parameter! variable is constant during simulation.  This prefix gives the library designer the possibility to express that the physical equations in a library are only valid if some of the used components are constant during simulation.  The same also holds for discrete-time and constant variables.  Additionally, the \lstinline!parameter! prefix allows a convenient graphical user interface in an experiment environment, to support quick changes of the most important constants of a compiled model.  In combination with an if-clause, a \lstinline!parameter! prefix allows removing parts of a model before the symbolic processing of a model takes place in order to avoid variable causalities in the model (similar to \lstinline!#ifdef! in C).  Class parameters can be sometimes used as an alternative.