Optional ReadonlycommandCommand words beyond the registry's (ChRIS: its plugins).
Optional ReadonlyoptionsThe --option words for a line, or null when the line is not one the backend completes.
Optional ReadonlypathWhether a command word of the backend's takes paths as its arguments (ChRIS: a /bin entry, whose operand is a path, #1305).
Optional ReadonlyrootNames completed at / even when the root's listing lacks them; absent, the core's usr.
What a backend adds to completion.