clarified session structure: avoid ambiguity of file ~~/src/HOL/Library/Old_Datatype.thy;

--- a/src/HOL/ROOT	Wed Apr 19 12:27:36 2017 +0200
+++ b/src/HOL/ROOT	Wed Apr 19 15:53:58 2017 +0200
@@ -20,7 +20,8 @@
   *}
   options [document = false, theory_qualifier = "HOL",
     quick_and_dirty = false, record_proofs = 2, parallel_proofs = 0]
-  theories "~~/src/HOL/Library/Old_Datatype"
+  sessions "HOL-Library"
+  theories "HOL-Library.Old_Datatype"
   files
     "Tools/Quickcheck/Narrowing_Engine.hs"
     "Tools/Quickcheck/PNF_Narrowing_Engine.hs"