I don't think I can actually do either of those things here, since this is the part int he code where I (finally) take advantage of the defaultdict's returning ```None``` when a value isn't present.  At this point in the code, ```merged_data``` contains all ''valid'' (sample id, metadata category) pairs, but we need to be able to pull out ```None``` when it tries to retrieve a (sample_id, metadata category) pair for which there is no value.