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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def top(self, N, mask=NotSpecified, groupby=NotSpecified):
 """ Construct a Filter matching the top N asset values of self each day. If ``groupby`` is supplied, returns a Filter matching the top N asset values for each group. Parameters N : int Number of assets passing the returned filter each day. mask : zipline.pipeline.Filter, optional A Filter representing assets to consider when computing ranks. If mask is supplied, top values are computed ignoring any asset/date pairs for which `mask` produces a value of False. groupby : zipline.pipeline.Classifier, optional A classifier defining partitions over which to perform ranking. Returns ------- filter : zipline.pipeline.filters.Filter """

        if N == 1:
            # Special case: if N == 1, we can avoid doing a full sort on every
            # group, which is a big win.
            return self._maximum(mask=mask, groupby=groupby)
        return self.rank(ascending=False, mask=mask, groupby=groupby) <= N