Closes #2270

Changing as decided; so that hold and backSample don't allow general expressions as arguments.

chapters/synchronous.tex

@@ -922,7 +922,8 @@ \subsection{Flattening of Model}\doublelabel{flattening-of-model}
 
 Furthermore, each non-trivial expression (non-literal, non-constant,
 non-parameter, non-variable), expr\textsubscript{i}, appearing as first
-argument of any clock conversion operator is recursively replaced by a
+argument of a clock conversion operator (except hold and backSample) 
+is recursively replaced by a
 unique variable, v\textsubscript{i}, and the equation v\textsubscript{i}
 = expr\textsubscript{i} is added to the equation set.