Add simple assignment statement to index

chapters/statements.tex

@@ -124,7 +124,7 @@ \section{Statements}\label{statements}
 
 \subsection{Simple Assignment Statements}\label{simple-assignment-statements}
 
-The syntax of simple assignment\index{assignment} statement is as follows:
+The syntax of \firstuse{simple assignment statement}\index{assignment statement!simple} is as follows:
 \begin{lstlisting}[language=grammar]
 component-reference ":=" expression
 \end{lstlisting}