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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 _csr_matrix_indices(S):
 """ Generate the indices of nonzero entries of a csr_matrix S """

    m, n = S.shape

    for i in range(m):
        for j in range(S.indptr[i], S.indptr[i+1]):
            row_index, col_index = i, S.indices[j]
            yield row_index, col_index