Clarify that a clock cannot be input to hold. (#2773)

Closes #2174

chapters/synchronous.tex

@@ -208,10 +208,14 @@ \subsection{Clocks and Clocked Variables}\label{clocks-and-clocked-variables}
   \caption{A clock variable.  The value of a clock variable is not defined -- the plot marks only indicate \emph{when} the clock is active.}\label{fig:clock-variable}
 \end{figure}
 
-\begin{definition}[Clocked variable]
+\begin{definition}[Clocked variable]\label{def:clocked-variable}
 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
+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.
+\begin{nonnormative}
+Note that clock variables are not included in this list.
+This implies that clock variables cannot be used where clocked variables are required.
+\end{nonnormative}
 
 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 \cref{fig:clocked-variable}.
@@ -585,7 +589,7 @@ \subsection{Base-clock conversion operators}\label{base-clock-conversion-operato
 hold($u$)
 \end{lstlisting}\end{synopsis}
 \begin{semantics}
-Input argument $u$ is a clocked component expression (\cref{def:component-expression}) or a parameter expression.
+Input argument $u$ is a clocked (\cref{def:clocked-variable}) component expression (\cref{def:component-expression}) or a parameter expression.
 The operator returns a piecewise constant signal of the same type as $u$.
 When the clock of $u$ ticks, the operator returns $u$ and otherwise returns the value of $u$ from the last clock activation.
 Before the first clock activation of $u$, the operator returns the start value of $u$, see \cref{initialization-of-clocked-partitions}.