'Construct and return new feature encoding, based on a given
training corpus ``train_toks``.
:type train_toks: list(tuple(dict, str))
:param train_toks: Training data, represented as a list of
pairs, the first member of which is a feature dictionary,
and the second of which is a classification label.'

def train(cls, train_toks):

    raise NotImplementedError()
