Presumably the permissions language is broadly defined and has something like filter = property, operand, value with few constraints on which operands can

Presumably the permissions language is broadly defined and has something like filter = property, operand, value with few constraints on which operands can be used in which situations, to keep parsing the language simple (parsers being notoriously prone to vulnerabilities after all). In retrospect perhaps that isn't a good trade-off, but it would be tricky to tighten things up now without breaking lots of existing users.