add header

--- a/src/HOL/Real/PReal.thy	Thu Sep 15 22:16:23 2005 +0200
+++ b/src/HOL/Real/PReal.thy	Thu Sep 15 23:16:04 2005 +0200
@@ -7,6 +7,8 @@
                   provides some of the definitions.
 *)
 
+header {* Positive real numbers *}
+
 theory PReal
 imports Rational
 begin