Use pure math instead of \lstinline for math-like content

As a variable in Modelica isn't referred to using explicit application to a time argument, the presentation of the different kinds of discrete-time variables isn't suitable for \lstinline.

chapters/synchronous.tex

@@ -199,9 +199,9 @@ \subsection{Clocks and Clocked Variables}\doublelabel{clocks-and-clocked-variabl
 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{discrete-time-expressions}.)  Variables $m(t)$ of base type \lstinline!Real!, \lstinline!Integer!, \lstinline!Boolean!, enumeration, and \lstinline!String! that are
+\emph{constant} inside each interval $t_{i} \leq t < t_{i+1}$ (i.e., piecewise constant continuous-time variables).  In other words, $m(t)$ \emph{changes}
+value \emph{only at events}: $m(t) = m(t_{i})$, 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}
 
@@ -213,7 +213,7 @@ \subsection{Clocks and Clocked Variables}\doublelabel{clocks-and-clocked-variabl
 \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
+Clock variables $c(t_{i})$ 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}
 
@@ -234,8 +234,8 @@ \subsection{Clocks and Clocked Variables}\doublelabel{clocks-and-clocked-variabl
 \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
+The elements of clocked variables $r(t_{i})$ are of base type \lstinline!Real!, \lstinline!Integer!, \lstinline!Boolean!, enumeration, \lstinline!String! that are associated uniquely with
+a clock $c(t_{i})$.  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