>>> from scipy.sparse import coo_matrix
>>> sigma_x = np.array([[0, 1], [1, 0]])
>>> sigma_y = np.array([[0, -1j], [1j, 0]])
>>> sp_x = graph.constant_sparse_pwc_operator(duration=0.1, operator=coo_matrix(sigma_x))
>>> sp_y = graph.constant_sparse_pwc_operator(duration=0.1, operator=coo_matrix(sigma_y))
>>> graph.sparse_pwc_sum([sp_x, sp_y])
<SparsePwc: operation_name="sparse_pwc_sum", value_shape=(2, 2)>