Merge pull request modelica#3214 from HansOlsson/PreviousParameter

Clarify previous parameter expression

chapters/synchronous.tex

@@ -522,33 +522,42 @@ \section{Clock Constructors}\label{clock-constructors}
 
 \section{Clocked State Variables}\label{clocked-state-variables}
 
+\begin{definition}[Clocked state variable]\label{def:clocked-state-variable}\index{clocked!state variable}\index{state variable!clocked}
+A component expression which is not a parameter, and to which \lstinline!previous! has been applied.
+\end{definition}
+
 The previous value of a clocked variable can be accessed with the \lstinline!previous! operator, listed below.
 \begin{center}
 \begin{tabular}{l|l l}
 \hline
 \tablehead{Expression} & \tablehead{Description} & \tablehead{Details}\\
 \hline
 \hline
-{\lstinline!previous($u$)!} & Previous value of clocked state variable & \Cref{modelica:previous} \\
+{\lstinline!previous($u$)!} & Previous value of clocked variable & \Cref{modelica:previous} \\
 \hline
 \end{tabular}
 \end{center}
 
-A variable to which \lstinline!previous! has been applied is called a \firstuse[clocked!state variable]{clocked state variable}\index{state variable!clocked}.
-
 \begin{operatordefinition}[previous]
 \begin{synopsis}\begin{lstlisting}
 previous($u$)
 \end{lstlisting}\end{synopsis}
 \begin{semantics}
-The input argument $u$ is a component expression (\cref{def:component-expression}) or a parameter expression.
-The return argument has the same type as the input argument.
+The input argument $u$ is a component expression (\cref{def:component-expression}).
+If $u$ is a parameter, its value is returned.
+
+Otherwise:
 Input and return arguments are on the same clock.
 At the first tick of the clock of $u$ or after a reset transition (see \cref{reset-handling}), the start value of $u$ is returned, see \cref{initialization-of-clocked-partitions}.
 At subsequent activations of the clock of $u$, the value of $u$ from the previous clock activation is returned.
 \end{semantics}
 \end{operatordefinition}
 
+\begin{nonnormative}
+At a clock tick only the (previous) values of the clocked state variables are needed to compute the new values of all clocked variables on that clock.
+This roughly corresponds to state variables in continuous time.
+\end{nonnormative}
+
 \section{Partitioning Operators}\label{partitioning-operators}
 
 A set of \emph{clock conversion operators} together act as boundaries