base root = "";

--- a/src/Pure/Thy/path.ML	Wed Nov 05 09:08:35 1997 +0100
+++ b/src/Pure/Thy/path.ML	Wed Nov 05 11:19:15 1997 +0100
@@ -36,6 +36,7 @@
   | absolute _ = false;
 
 fun base (Path []) = ""
+  | base (Path ["/"]) = ""
   | base (Path xs) = snd (split_last xs);