What, exactly, is the relationship between keys, URLs and URIs?  As I understand it, for each key, git-annex keeps a list of zero or more URLs/URIs from which the key's contents may be downloaded.  Is each entry in this list specific to one special remote, i.e. is this really a list of pairs (URL/URI, special-remote-uuid)? Is the record of which key is present in which remote completely independent of the record of which key may be downloaded from which remote through which URL/URI?  Is the only difference between URLs and URIs that, when a URL is added for a key, git-annex records the key as present in the web special remote, while when a URI is added, it doesn't?   (Sorry for too many questions.)