Use a frozenset for use inside binarytree._eval_use_flags().

svn path=/main/trunk/; revision=15407
diff --git a/pym/portage/dbapi/bintree.py b/pym/portage/dbapi/bintree.py
index ce162fc..4420b72 100644
--- a/pym/portage/dbapi/bintree.py
+++ b/pym/portage/dbapi/bintree.py
@@ -1046,7 +1046,7 @@
 		return False
 
 	def _eval_use_flags(self, cpv, metadata):
-		use = metadata["USE"].split()
+		use = frozenset(metadata["USE"].split())
 		raw_use = use
 		iuse = set(f.lstrip("-+") for f in metadata["IUSE"].split())
 		use = [f for f in use if f in iuse]