I was trying to make a segment of users who didn't link their accounts to any of my game's login providers.
I know I could hack it using tags, but It seems as easy as doing a filter like this:
(NOT Linked user account to Facebook) AND (NOT Linked user account to GooglePlay)
In fact, it could be useful to use the same queries that are available on Players search.
Now we can only use AND and OR operators (in a very restrictive way).
I think the NOT operator could be a really nice addition.