def test_infuse():
    ...
    # test without parameters
    # test with only rate
    # test with only volume
    # test with both
    # test with too large volume


def test_stop():
    ...
    # run, stop, is-pumping is False


def test_withdraw():
    ...
    # test without parameters
    # test with only rate
    # test with only volume
    # test with both
    # test with too large volume
