load variable.ML before conv.ML;

--- a/src/Pure/ROOT.ML	Thu Oct 04 16:59:28 2007 +0200
+++ b/src/Pure/ROOT.ML	Thu Oct 04 16:59:29 2007 +0200
@@ -64,9 +64,9 @@
 use "pure_thy.ML";
 use "display.ML";
 use "drule.ML";
-use "conv.ML";
 use "morphism.ML";
 use "variable.ML";
+use "conv.ML";
 use "tctical.ML";
 use "search.ML";
 use "tactic.ML";