Bugfix in alpha-covnersion.

diff --git a/data2.py b/data2.py
index f4231c2..43085f5 100644
--- a/data2.py
+++ b/data2.py
@@ -144,10 +144,11 @@ def rename_off(obj,excl):
   old = copy(obj)
   old_e = Set(*excl._list)
   for v in CV(obj):
-    nv = first_var(excl + FV(obj))
+    nv = first_var(excl + V(obj))
+    debug("%s: renaming %s to %s" % (d_show(obj),v,nv))
     obj = obj.rename(v,Var(nv))
     excl.add(Var(nv))
-  debug("%s: renaming off %s => %s" % (d_show(old),old_e,d_show(obj)))
+#   debug("%s: renaming off %s => %s" % (d_show(old),old_e,d_show(obj)))
   return obj

 def alpha(obj,app,excl_vars=Set()):