I wanted to create a playlist of only unrated songs. Since rating is a numeric field, there's no option for blank (not the issue).
I built rule that required NONE of |rating <= 5|, |rating >= 1|, which worked just fine. However, after quitting play, I couldn't restart; console showed an error parsing the format string "rating >= 1 NOT rating <= 5".
A rule that simply checked for NONE of |rating >= 1| would be better, but NONE isn't available until you add multiple fields.
I changed the rule in the sqlite database to simply read "NOT rating >= 1", and Play did the smart thing... it sees a NONE rule with 1 field, and pulls the right stuff from the library.