--- a/ProgTutorial/Essential.thy	Wed Jun 20 08:29:12 2012 +0100
+++ b/ProgTutorial/Essential.thy	Wed Jun 20 08:53:38 2012 +0100
@@ -1966,8 +1966,8 @@
 
  
 text {*
-  One often wants to know the theorems that are used in the proof 
-  of a theorem. They can be obtained by introspecting the theorem.
+  One often wants to know the theorems that are involved in proving 
+  a theorem. They can be obtained by introspecting the theorem.
   To introspect a theorem, let us define the following three functions that 
   analyse the @{ML_type_ind proof_body} data-structure from the structure 
   @{ML_struct Proofterm}.