def METHOD_NAME(mrcs_data):
    # Each line of the txt file points to an .mrcs file with 1000 particles.
    # Specify indices that span these files.
    src = ImageSource.from_file(
        f"{DATA_FOLDER}/toy_projections_2.txt",
        indices=np.array([35, 631, 1531, 363, 1693, 1875]),
        lazy=False,
    )
    assert src.shape == (6, 30, 30)  # Not (100, 30, 30)
    # Once read, caller-side indexing can be done as usual (i.e. going from 0 to src.n),
    # and we can forget about what indices were originally passed in.
    data = src.images(np.array([2, 3, 5, 0, 1, 4]))
    assert torch.allclose(
        mrcs_data[np.array([531, 363, 875, 35, 631, 693]), :, :], data
    )