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s/subset/superset/ :) (any JSON document is a valid YAML document, allegedly)
Just a note -- there are differences in the output between the previous YAML outputter and the JSON outputter, but I think it's actually a bug in the old YAML generator. :)
The YAML outputter incorrectly turned query.pages from a map into a vector array, probably because it saw integer keys but didn't actually vet them. (Ah PHP, your frankenstein hashmap+array construct strikes again!) It's possible that YAML-using clients that assumed the broken structure may not always work with the new, HOWEVER the new output will actually be the correct output structure. So that's probably good.