'Return the position of the specified key in the OrderedDict.
>>> d = OrderedDict(((1, 3), (3, 2), (2, 1)))
>>> d.index(3)
1
>>> d.index(4)
Traceback (most recent call last):
ValueError: list.index(x): x not in list'

def index(self, key):

    return self._sequence.index(key)
