in theory `tag` is for "association by reference". you reference other nodes, which can be either objects or links. but what's the best way to reference an arbitrary string? there is the unofficial extension of a `Hashtag` but this ends up getting used as a signal to show your tag alongside other posts that share the same tag. also the format of hashtags is a little weird -- no one can really agree on it. does it contain the hash sign? is it allowed to contain spaces? is it a subtype of Link, or a subtype of Object? should it or should it not appear in the content, since `tag` seems to be used to markup microsyntaxes?