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

chapters/classes.tex

@@ -341,14 +341,15 @@ \subsection{Component Variability Prefixes discrete, parameter, constant}\label{
 \begin{itemize}
 \item
   A variable \lstinline!vc! declared with \lstinline!constant!\indexinline{constant} prefix remains constant during transient analysis, with a value that is unaffected by the initialization problem.
-  This is called a constant, or constant variable.
+  This is called a \firstuse{constant}, or \firstuse{constant variable}\index{constant variable}\index{component variability!constant}.
 \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.
-  This is called a parameter, or parameter variable.
+  This is called a \firstuse{parameter}, or \firstuse{parameter variable}\index{parameter variable}\index{component variability!parameter}.
 \item
   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.
+  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.
+  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} variable is a \lstinline!Real! variable without any prefix that is not assigned in a \lstinline!when!-clause.

chapters/equations.tex

@@ -718,7 +718,7 @@ \section{Initialization, initial equation, and initial algorithm}\label{initiali
 There is no special handling of inactive \lstinline!when!-statements during initialization, instead variables assigned in \lstinline!when!-statements are initialized using \lstinline!v := pre(v)! before the body of the algorithm (since they are discrete), see \cref{execution-of-an-algorithm-in-a-model}.
 \end{nonnormative}
 
-Further constraints, necessary to determine the initial values of all variables (depending on the variability of the variable, see \cref{component-variability-prefixes-discrete-parameter-constant} for definitions), can be defined in the following ways:
+Further constraints, necessary to determine the initial values of all variables (depending on the component variability, see \cref{component-variability-prefixes-discrete-parameter-constant} for definitions), can be defined in the following ways:
 \begin{enumerate}
 \item
   As equations in an \lstinline!initial equation!\indexinline{initial equation} section or as assignments in an \lstinline!initial algorithm!\indexinline{initial algorithm} section.