Update synchronous data flow and single assignment rule to be correct.

Closes #2348

chapters/equations.tex

@@ -576,22 +576,21 @@ \section{Synchronous Data-flow Principle and Single Assignment Rule}\doublelabel
 assignment rule, which are defined in the following way:
 
 \begin{enumerate}
-\item All variables keep their actual values until these values are
-explicitly changed. Variable values can be accessed at any time instant
-during continuous integration and at event instants.
+\item Discrete variables keep their actual values until these variables are explicitly changed.
+Differentiated variables have \lstinline!der(x)! corresponding to the time-derivative of \lstinline!x!, 
+and \lstinline!x! is continuous, except when reinit is triggered - see \autoref{reinit}. 
+Variable values can be accessed at any time instant during continuous integration and at event instants.
 
-\item At every time instant, during continuous integration and at event
-instants, the active equations express relations between variables which
-have to be fulfilled concurrently (equations are not active if the
-corresponding if-branch, when-clause or block in which the equation is
-present is not active).
+\item At every time instant, during continuous integration and at event instants, 
+the equations express relations between variables which have to be fulfilled concurrently..
 
 \item  Computation and communication at an event instant does not take time.
 {[}\emph{If computation or communication time has to be simulated, this
 property has to be explicitly modeled}{]}.
 
-\item  The total number of equations is identical to the total ``number of
-unknown variables'' (= single assignment rule).
+\item  There must exist a perfect matching of variables to equations after flattening 
+(perfect matching rule - previously called single assignment rule); 
+see also globally balanced \autoref{balanced-models}.
 \end{enumerate}
 
 \section{Events and Synchronization}\doublelabel{events-and-synchronization}