def METHOD_NAME(
    maximum_unused_protocols: int,
    existing_protocols: List[_ProtocolInfo],
    expected_deletion_plan: Set[str],
) -> None:
    """It should return a plan that leaves at least one slot open for a new protocol."""
    subject = ProtocolDeletionPlanner(maximum_unused_protocols=maximum_unused_protocols)
    result = subject.plan_for_new_protocol(existing_protocols=existing_protocols)
    assert result == expected_deletion_plan