'Given a (featureset, label) pair, return the corresponding
vector of joint-feature values.  This vector is represented as
a list of ``(index, value)`` tuples, specifying the value of
each non-zero joint-feature.
:type featureset: dict
:rtype: list(tuple(int, int))'

def encode(self, featureset, label):

    raise NotImplementedError()
