Revert ca9f22e as part of modelica#2923 to more clearly define discre…

…te-time for variables.

chapters/operatorsandexpressions.tex

@@ -1475,7 +1475,7 @@ \subsection{Discrete-Time Expressions}\label{discrete-time-expressions}
 For a scalar equation, the rule follows from the observation that a discrete-valued equation does not provide sufficient information to solve for a continuous-valued variable.
 Hence, and according to the perfect matching rule (see \cref{synchronous-data-flow-principle-and-single-assignment-rule}), such an equation must be used to solve for a discrete-valued variable.
 By the interpretation of \eqref{eq:dae-discrete-valued} in \cref{modelica-dae-representation}, it follows that one of \lstinline!expr1! and \lstinline!expr2! must be the variable, and the other expression its solution.
-Since a discrete-valued variable is a discrete-time variable, it follows that its solution on the other side of the equation must have at most discrete-time variability.
+Since a discrete-valued variable is a discrete-time expression, it follows that its solution on the other side of the equation must have at most discrete-time variability.
 That is, both sides of the equation are discrete-time expressions.
 
 For example, this rule shows that (outside of a \lstinline!when!-clause) \lstinline!noEvent! cannot be applied to either side of a \lstinline!Boolean!, \lstinline!Integer!, \lstinline!String!, or \lstinline!enumeration! equation, as this would result in a non-discrete-time expression.