rr_graph: Improve error checking around pass gates.

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>

v2x/lib/asserts.py

diff --git a/v2x/lib/asserts.py b/v2x/lib/asserts.py
index fdbf2db..a1e28f2 100644
--- a/v2x/lib/asserts.py
+++ b/v2x/lib/asserts.py
@@ -23,6 +23,12 @@
     assert a == b, msg or __safe_error("!=", a, b)
 
 
+def assert_not_in(needle, haystack, msg=None):
+    """Assert equal with better error message."""
+    assert needle not in haystack, msg or __safe_error("already in", needle,
+                                                       haystack)
+
+
 def assert_is(a, b):
     """Assert is with better error message."""
     assert a is b, __safe_error("is not", a, b)