ENH: add __contains__ to Cycler object

cycler.py

@@ -135,6 +135,9 @@ def __init__(self, left, right=None, op=None):
         self._keys = _process_keys(self._left, self._right)
         self._op = op
 
+    def __contains__(self, k):
+        return k in self._keys
+
     @property
     def keys(self):
         """

test_cycler.py

@@ -328,3 +328,18 @@ def test_by_key_mul():
     assert_equal(input_dict['lw'] * len(input_dict['c']),
                  res['lw'])
     yield _by_key_helper, cy
+
+
+def test_contains():
+    a = cycler('a', range(3))
+    b = cycler('b', range(3))
+
+    assert 'a' in a
+    assert 'b' in b
+    assert 'a' not in b
+    assert 'b' not in a
+
+    ab = a+b
+
+    assert 'a' in ab
+    assert 'b' in ab