CREATE TABLE annotation_events (
 id INTEGER PRIMARY KEY,
 request_id TEXT NOT NULL UNIQUE,
 paper_id INTEGER NOT NULL REFERENCES papers(id),
 original_paper_id INTEGER NOT NULL,
 kind TEXT NOT NULL CHECK(kind IN ('tag','research_type')),
 namespace TEXT NOT NULL, value TEXT NOT NULL,
 source_record_id INTEGER REFERENCES source_records(id),
 evidence_key TEXT NOT NULL, evidence_url TEXT,
 reason TEXT NOT NULL, actor TEXT NOT NULL, observed_at TEXT NOT NULL,
 retracts_id INTEGER UNIQUE REFERENCES annotation_events(id),
 CHECK(retracts_id IS NULL OR retracts_id<>id)
);
CREATE INDEX annotation_events_paper ON annotation_events(paper_id,kind,id);
CREATE INDEX annotation_events_label ON annotation_events(kind,namespace,value,paper_id);