added system, ind_defs, axclass;

--- a/doc/Contents	Mon May 12 18:43:24 1997 +0200
+++ b/doc/Contents	Mon May 12 18:44:43 1997 +0200
@@ -1,3 +1,6 @@
-intro	Introduction to Isabelle
-ref	The Isabelle Reference Manual
-logics  Isabelle's Object-Logics
+intro		Introduction to Isabelle
+ref		The Isabelle Reference Manual
+system		The Isabelle System Manual
+logics		Isabelle's Object-Logics
+ind-defs	(Co)Inductive Definitions in ZF
+axclass		Tutorial on Axiomatic Type Classes