Clarify impure in systems of equations.

chapters/functions.tex

@@ -232,6 +232,7 @@ \section{Pure Modelica Functions}\label{pure-modelica-functions}
   If a function is declared as \lstinline!impure! any function extending from it shall be declared as \lstinline!impure!.
 \item
   A deprecated semantics is that external functions (and functions defined in Modelica directly or indirectly calling them) without \lstinline!pure! or \lstinline!impure! keyword are assumed to be impure, but without any restriction on calling them.
+  Similarly they can be part of systems of equations without restrictions.
   Except for the function \lstinline!Modelica.Utilities.Streams.print!, a diagnostic must be given if called in a simulation model.
 \end{itemize}
 
@@ -295,8 +296,8 @@ \section{Pure Modelica Functions}\label{pure-modelica-functions}
 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.
+Comment: The semantics are undefined if the function call of an impure function is part of an algebraic loop.
+This can occur for the deprecated case of external functions assumed to be impure, or when using \lstinline!pure(expression)! to call an impure function in a pure function.
 \end{nonnormative}
 
 \begin{example}