'Sets value at each (i, j) to zero, preserving sparsity structure.
Here (i,j) index major and minor respectively.'

def _zero_many(self, i, j):

    (i, j, M, N) = self._prepare_indices(i, j)
    n_samples = len(i)
    offsets = np.empty(n_samples, dtype=self.indices.dtype)
    ret = _sparsetools.csr_sample_offsets(M, N, self.indptr, self.indices, n_samples, i, j, offsets)
    if (ret == 1):
        self.sum_duplicates()
        _sparsetools.csr_sample_offsets(M, N, self.indptr, self.indices, n_samples, i, j, offsets)
    self.data[offsets[(offsets > (-1))]] = 0
