diff -r 8ad407e77ea0 -r ae2f0b40c840 ProgTutorial/Parsing.thy
--- a/ProgTutorial/Parsing.thy	Tue Nov 17 18:40:11 2009 +0100
+++ b/ProgTutorial/Parsing.thy	Tue Nov 17 20:36:18 2009 +0100
@@ -674,6 +674,22 @@
   @{ML_file "Pure/General/position.ML"}.
   \end{readmore}
 
+  \begin{exercise}\label{ex:contextfree}
+  Write a parser for the context-free grammar representing arithmetic 
+  expressions with addition and multiplication. As usual, multiplication 
+  binds stronger than addition, and both of them nest to the right. 
+  The context-free grammar is defined as:
+
+  \begin{center}
+  \begin{tabular}{lcl}
+  @{text "<Basic>"}  & @{text "::="} & @{text "<Number> | (<Expr>)"}\\
+  @{text "<Factor>"} & @{text "::="} & @{text "<Basic> * <Factor> | <Basic>"}\\
+  @{text "<Expr>"}   & @{text "::="} & @{text "<Factor> + <Expr>  | <Factor>"}\\
+  \end{tabular}
+  \end{center}
+
+  Hint: Be careful with recursive parsers.
+  \end{exercise}
 *}
 
 section {* Parsers for ML-Code (TBD) *}