Folks: so Stas and I've spent two days talking to the wikidata folks and I've learned some fun stuff. I'm sending this email to this mailing list, because, well, I just have to write this down and this list cares about things. Maybe they'll care about this thing. I'm not sending this to wikidata's mailing list because I'm not really confident enough to send this to experts yet.
Anyway, the most interesting bit that I learned so far is that the Wikidata team claims that Wikidata doesn't describe truth. That might seem like a silly difference at first but you start to get into trouble when you want to query it and don't understand that. Think about it this way: there are multiple values for the Jesus's birthday https://www.wikidata.org/wiki/Q302#P569 and wikidata actually doesn't claim that either of them are true, just that they are *according to some sources*. Look also at George Washington's spouse https://www.wikidata.org/wiki/Q23#P26. She has a qualifier - the date of their marriage. These qualifies are like preconditions to the truth. Kinda. They aren't always used that way but you can sort of pretend.
But we can emerge from Cartesian doubt! Wikidata has some concept of "true enough for most uses" called "best rank" https://www.wikidata.org/wiki/Help:Ranking. Its a reasonably simple concept that amounts to "the community decides". So the plan is to implement queries against that first. This should be good enough for wikigrok initially and faster to implement and query because it allows us to ignore things like qualifiers and references.
Nik