Use 'definition' environment for the different kinds of discrete-time…

… variables

It hurts a little to break the table apart, but I think it has to be done in the spirit of consistency.

chapters/synchronous.tex

@@ -184,70 +184,60 @@ \section{Definitions}\doublelabel{definitions}
 
 \subsection{Clocks and Clocked Variables}\doublelabel{clocks-and-clocked-variables}
 
-In \autoref{discrete-time-expressions} the term \emph{discrete-time} Modelica expression and in
-\autoref{continuous-time-expressions} the term \emph{continuous-time} Modelica expression is
-defined. In this chapter, two additional kinds of discrete-time
-expressions/variables are defined that are associated to clocks and are
-therefore called \firstuse{clocked discrete-time} expressions:
-% henrikt-ma: Misusing \tablehead inside the table below to act as some sort of table title.
-% Similarly, while this table is actually a list of definitions, the styling of the terms being described
-% should probably use something else than \firstuse, perhaps more like a 'description' environment.
-% This is all different than how things are done elsewhere, but fixing this is considered a topic for issue #2562.
-\begin{longtable}[]{|p{7.3cm}|p{7.3cm}|}
-\hline
-\multicolumn{2}{|p{14.6cm}|}{\tablehead{The different kinds of discrete-time variables in Modelica}}\\ \hline
-\endhead
-\begin{tabular}{p{7cm}}
-\includegraphics[width=3in,height=1.875in]{piecewise}
-\end{tabular}&\begin{tabular}{p{7cm}}\firstuse{Piecewise-constant variables}\\
-(See \autoref{discrete-time-expressions}.)  Variables \lstinline!m(t)! of base type \lstinline!Real!, \lstinline!Integer!, \lstinline!Boolean!,
-enumeration, and \lstinline!String! that are \emph{constant} inside each interval
-t\textsubscript{i} $\le$ t \textless{} t\textsubscript{i+1} (= piecewise
-constant continuous-time variables). In other words, \lstinline!m(t)!
-\emph{changes} value \emph{only at events}. This means, \lstinline!m($t$)! =
-\lstinline!m($t_{i}$)!, for $t_{i} \leq t < t_{i+1}$. Such variables depend continuously on time and
-they are discrete-time variables.
-\end{tabular}\\ \hline
-\begin{tabular}{p{7cm}}
-\includegraphics[width=3in,height=1.875in]{clock}
-\end{tabular}&
-\begin{tabular}{p{7cm}}\firstuse{Clock variables}\\
-Clock variables \lstinline!c($t_{i}$)! are of base type \lstinline!Clock!. A
-clock is either defined by a constructor (such as \lstinline!Clock(3)!)
-that defines when the clock ticks (is active) at a particular time
-instant, or it is defined with clock operators relatively to other
-clocks, see \autoref{base-clock-conversion-operators}.
-\par
-\begin{example*}
+In \autoref{discrete-time-expressions} the term \emph{discrete-time} Modelica expression and in \autoref{continuous-time-expressions} the term \emph{continuous-time} Modelica expression is defined.
+In this chapter, two additional kinds of discrete-time expressions/variables are defined that are associated to clocks and are therefore called \firstuse{clocked discrete-time} expressions.  The
+different kinds of discrete-time variables in Modelica are defined below.
+
+\begin{definition}[Piecewise-constant variable]
+(See \autoref{discrete-time-expressions}.)  Variables \lstinline!m(t)! of base type \lstinline!Real!, \lstinline!Integer!, \lstinline!Boolean!, enumeration, and \lstinline!String! that are
+\emph{constant} inside each interval t\textsubscript{i} $\le$ t \textless{} t\textsubscript{i+1} (= piecewise constant continuous-time variables).  In other words, \lstinline!m(t)! \emph{changes}
+value \emph{only at events}.  This means, \lstinline!m($t$)! = \lstinline!m($t_{i}$)!, for $t_{i} \leq t < t_{i+1}$.  Such variables depend continuously on time and they are discrete-time variables.
+See \autoref{fig:piecewise-constant-variable}.
+\end{definition}
+
+\begin{figure}[H]
+  \begin{center}
+    \includegraphics[width=3in]{piecewise}
+  \end{center}
+  \caption{A piecewise-constant variable.}\doublelabel{fig:piecewise-constant-variable}
+\end{figure}
+
+\begin{definition}[Clock variable]
+Clock variables \lstinline!c($t_{i}$)! are of base type \lstinline!Clock!.  A clock is either defined by a constructor (such as \lstinline!Clock(3)!) that defines when the clock ticks (is active) at
+a particular time instant, or it is defined with clock operators relatively to other clocks, see \autoref{base-clock-conversion-operators}.  See \autoref{fig:clock-variable}.
+\end{definition}
+
+\begin{example}
+Clock variables:
 \begin{lstlisting}[language=modelica]
-  Clock c1 = Clock($\ldots$);
-  Clock c2 = c1;
-  Clock c3 = subSample(c2, 4);
+Clock c1 = Clock($\ldots$);
+Clock c2 = c1;
+Clock c3 = subSample(c2, 4);
 \end{lstlisting}
-\end{example*}
-\end{tabular}\\ \hline
-\begin{tabular}{p{7cm}}
-\includegraphics[width=3in,height=1.875in]{clocked}
-\end{tabular}&
-\begin{tabular}{p{7cm}}\firstuse{Clocked variables}\\
-The elements of clocked variables \lstinline!r($t_{i}$)! are of
-base type \lstinline!Real!, \lstinline!Integer!, \lstinline!Boolean!, enumeration, \lstinline!String! that are
-associated uniquely with a clock \lstinline!c($t_{i}$)!. A
-clocked variable can only be directly accessed at the event instant
-where the associated clock is active. A constant and a
-parameter can always be used at a place where a clocked variable is
-required.
-
-At time instants where the associated clock is not active, the value of
-a clocked variable can be inquired by using an explicit cast operator,
-see below. In such a case \lstinline!hold! semantics is used, in other words
-the value of the clocked variable from the last event instant is used.
-\par
-\begin{nonnormative*}
-This is visualized in the left figure with the dashed green lines.
-\end{nonnormative*}
-\end{tabular}\\ \hline
-\end{longtable}
+\end{example}
+
+\begin{figure}[H]
+  \begin{center}
+    \includegraphics[width=3in]{clock}
+  \end{center}
+  \caption{A clock variable.}\doublelabel{fig:clock-variable}
+\end{figure}
+
+\begin{definition}[Clocked variable]
+The elements of clocked variables \lstinline!r($t_{i}$)! are of base type \lstinline!Real!, \lstinline!Integer!, \lstinline!Boolean!, enumeration, \lstinline!String! that are associated uniquely with
+a clock \lstinline!c($t_{i}$)!.  A clocked variable can only be directly accessed at the event instant where the associated clock is active.  A constant and a parameter can always be used at a place
+where a clocked variable is required.
+
+At time instants where the associated clock is not active, the value of a clocked variable can be inquired by using an explicit cast operator, see below.  In such a case \lstinline!hold! semantics is
+used, in other words the value of the clocked variable from the last event instant is used.  See \autoref{fig:clocked-variable}.
+\end{definition}
+
+\begin{figure}[H]
+  \begin{center}
+    \includegraphics[width=3in]{clocked}
+  \end{center}
+  \caption{A clocked variable.  The constant extrapolation of the value at the last event instant is illustrated with dashed green lines.}\doublelabel{fig:clocked-variable}
+\end{figure}
 
 \subsection{Base-Clock and Sub-Clock Partitions}\doublelabel{base-clock-and-sub-clock-partitions}