Describe 'partial' in text and add to index

chapters/classes.tex

@@ -155,6 +155,9 @@ \subsection{Component Declaration Static Semantics}\label{component-declaration-
 
 If the \lstinline[language=grammar]!type-specifier! of the component declaration denotes a built-in type (\lstinline!RealType!, \lstinline!IntegerType!, etc.), the flattened or instantiated component has the same type.
 
+A class defined with \lstinline!partial!\index{partial@\indexinline{partial}} in the \lstinline[language=grammar]!class-prefixes! is called a \firstuse{partial} class.
+Such a class is allowed to be incomplete, and cannot be instantiated in a simulation model; useful, e.g., as a base-class.
+
 If the \lstinline[language=grammar]!type-specifier! of the component does not denote a built-in type, the name of the type is looked up (\cref{static-name-lookup}).
 The found type is flattened with a new environment and the partially flattened enclosing class of the component.
 It is an error if the type is partial in a simulation model, or if a simulation model itself is partial.