'Parameters
user_id: int or string
User for which recommendations are to be computed.
how_many: int
Desired number of recommendations
rescorer:  function, optional
Rescoring function to apply before final list of
recommendations.
Returns
Return a list of recommended items, ordered from most strongly
recommend to least.'

def recommend(self, user_id, how_many, **params):

    raise NotImplementedError('BaseRecommender    is    an    abstract    class.')
