1 files changed, 15 insertions, 0 deletions
diff --git a/category.py b/category.py
index 61458dc..3acc16e 100644
--- a/category.py
+++ b/category.py
@@ -47,6 +47,21 @@ class Category:
             if args[key].nonempty():
                 retval.add(Category(self.name, **args))
 
+    def inverse_set(self):
+        """
+        Returns a set of Categories that will match any state NOT matched by
+        this one (provided it specifies the same or more arguments).
+        """
+        retval = set()
+        for key, value in self.args.iteritems():
+            value = value.complement()
+            if not value.nonempty():
+                continue
+            newargs = self.args.copy()
+            newargs[key] = value
+            retval.add(Category(self.name, **newargs))
+        return retval
+
     def intersect(self, other):
         """
         Returns a new category consisting only of states that are in this