added dummy set_session;

--- a/src/Pure/Thy/ROOT.ML	Tue Mar 18 17:03:05 1997 +0100
+++ b/src/Pure/Thy/ROOT.ML	Tue Mar 18 17:03:55 1997 +0100
@@ -35,3 +35,9 @@
     "ThmDB.thm_db := !thm_db;",
     "open ThmDB ReadThy;",
     "read_thy_reader_files ();"];
+
+
+(* FIXME tmp *)
+
+fun set_session s =
+  writeln ("This is the " ^ quote s ^ " session.");