Clarify impure in systems of equations. (#3571)

* Clarify impure in systems of equations.
* Update chapters/functions.tex
Co-authored-by: Henrik Tidefelt <henrikt@wolfram.com>

chapters/functions.tex

@@ -284,6 +284,11 @@ \section{Pure Modelica Functions}\label{pure-modelica-functions}
 The reason is that solving systems of equations generally requires expressions to be evaluated an unknown number of times.
 This includes the special handling of \lstinline!when initial()! during initialization.
 
+There are two ways in which an impure function could be called in a system of equations, namely in the deprecated case of external functions assumed to be impure, and when using \lstinline!pure($\ldots$)! to call an impure function from within a pure function.
+The side-effect semantics of the function call are then undefined.
+Specifically, the number of calls with external side-effects is unspecified.
+However, for impure functions where the outputs only depend on the inputs the system of equations should be solved correctly.
+
 \begin{nonnormative}
 A tool is not allowed to perform any optimizations on function
 calls to an impure function, e.g., reordering calls from different
@@ -294,11 +299,6 @@ \section{Pure Modelica Functions}\label{pure-modelica-functions}
 By \cref{function-compatibility-or-function-subtyping-for-functions}, it follows that an impure function can only be passed as argument to a function formal parameter of impure type.
 A function having a formal function parameter that is \lstinline!impure! must be marked \lstinline!pure! or \lstinline!impure!.
 
-\begin{nonnormative}
-Comment: The semantics are undefined if the function call of an
-impure function is part of an algebraic loop.
-\end{nonnormative}
-
 \begin{example}
 \begin{lstlisting}[language=modelica]
 function evaluateLinear // pure function