Clarify variability of functions.

Closes #2538

chapters/functions.tex

@@ -116,6 +116,29 @@ \subsection{Inheritance of Functions}\label{inheritance-of-functions}
 For example, it is possible to modify and extend a \lstinline!function! class to add default values for input variables.
 \end{nonnormative}
 
+A special case is defining a \lstinline!function! as a short-class definition with modifiers for inputs inside a model.
+These default values, unless overridden in the function call, will then be considered for variability similarly as if they were given in the function call.
+
+\begin{example}
+Demonstrating the variability implications.
+Note that functions cannot directly use non-constants in enclosing scopes, so we cannot write \lstinline!input Real x1=x;! directly in \lstinline!foo!.
+\begin{lstlisting}[language=modelica]
+model M
+  function foo
+    input Real x1;
+    input Real x2=2;
+    output Real y;
+  algorithm
+    y:=x1+x2;
+  end foo;
+  Real x=time;
+  function f1=foo(x1=x);
+  constant Real z=f1(x2=1); // Illegal since 'x' is seen as argument
+  constant Real z2=f1(x1=2); // Legal, since 'x1' has a new value.
+end M;
+\end{lstlisting}
+\end{example}
+
 \section{Function as a Specialized Class}\label{function-as-a-specialized-class}
 
 The function concept in Modelica is a specialized class (\cref{specialized-classes}).