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what gets marked depends on the system
marked: the object (accusative)
unmarked: the subject — the doer is the default
the dog runs.
S — intransitive subject, unmarked in both systems
the dog sees the cat.
A → O — transitive
she slept.
S — intransitive, unmarked
she wrote a letter.
A → O — transitive
the wind opens the door.
A → O — transitive
the door opens.
S — same word, same marking as the object above. in ergative alignment, this is the natural pairing: S and O together