'Retrieve the column corresponding to key in a Formula.
Parameters
key : Factor key
one of the Factor\'s keys
Returns
ndarray corresponding to key, when evaluated in current namespace'

def __getitem__(self, key):

    if (not self.ordinal):
        i = self.names().index(('(%s==%s)' % (self.termname, str(key))))
        return self()[i]
    else:
        v = self.namespace[self._name]
        return np.array([(vv == key) for vv in v]).astype(np.float)
