Update chapters/classes.tex

Co-authored-by: Henrik Tidefelt <henrikt@wolfram.com>

chapters/classes.tex

@@ -843,7 +843,15 @@ \subsection{Short Class Definitions}\label{short-class-definitions}
 
 \subsection{Combining Base Classes and Other Elements}\label{restriction-on-combining-base-classes-and-other-elements}\label{combining-base-classes-and-other-elements}
 
-It is not legal to combine equations, algorithms, components, non-empty base classes (see \cref{empty-class}), or protected elements with an extends from an array class, a class with non-empty \lstinline[language=grammar]!base-prefix!, a \firstuse{simple type} (\lstinline!Real!, \lstinline!Boolean!, \lstinline!Integer!, \lstinline!String! and enumeration types), or any class transitively extending from an array class, a class with non-empty \lstinline[language=grammar]!base-prefix!, or a simple type.
+It is not legal to combine equations, algorithms, components, non-empty base classes (see below), or protected elements with an extends from an array class, a class with non-empty \lstinline[language=grammar]!base-prefix!, a \firstuse{simple type} (\lstinline!Real!, \lstinline!Boolean!, \lstinline!Integer!, \lstinline!String! and enumeration types), or any class transitively extending from an array class, a class with non-empty \lstinline[language=grammar]!base-prefix!, or a simple type.
+
+\begin{definition}[Empty class]\index{empty-class}
+A class without equations, algorithms, or components, and where any base-classes are themselves empty.
+\end{definition}
+
+\begin{nonnormative}
+An empty class may contain annotations, such as graphics, and can be used more freely as base-class than other classes.
+\end{nonnormative}
 
 \begin{example}
 \begin{lstlisting}[language=modelica]