'Reduce nonzeros with a ufunc over the minor axis when non-empty
Warning: this does not call sum_duplicates()
Returns
major_index : array of ints
Major indices where nonzero
value : array of self.dtype
Reduce result for nonzeros in each major_index'

def _minor_reduce(self, ufunc):

    major_index = np.flatnonzero(np.diff(self.indptr))
    value = ufunc.reduceat(self.data, downcast_intp_index(self.indptr[major_index]))
    return (major_index, value)
