Now as example

Based on the reformulation of "over-optimize", this has become more of a typical example than a non-normative explanation.

chapters/functions.tex

@@ -1183,10 +1183,10 @@ \subsection{Using the Derivative Annotation}\label{using-the-derivative-annotati
 \begin{nonnormative}
 This means that the most restrictive derivatives should be written first.
 \end{nonnormative}
-\begin{nonnormative}
-A straightforward way of explaining a valid derivative is given by this model.
-The function value, \lstinline!x1!, should up to numerical precision match the integral of the derivative, \lstinline!x2!.
-Note that tools are not required to use the provided derivative, and might solve the equations completely without numeric integration.
+
+\begin{example}
+The following model illustrates the requirement that a provided derivative must be valid.
+That \lstinline!fder! is a valid derivative of \lstinline!f! means that it can be used safely to compute \lstinline!x2! by numeric integration: the function value, \lstinline!x1!, will up to numerical precision be matched by the integral of the derivative, \lstinline!x2!.
 \begin{lstlisting}[language=modelica]
   function f
     input Real x;
@@ -1205,7 +1205,8 @@ \subsection{Using the Derivative Annotation}\label{using-the-derivative-annotati
     x2=x1;
   end M;
 \end{lstlisting}
-\end{nonnormative}
+Note that tools are not required to use the provided derivative, and might solve the equations completely without numeric integration.
+\end{example}
 
 \begin{example}
 Use of \lstinline!order! to specify a second order derivative: