Was a stock in an index on a date

Point-in-time index membership with a four-state answer, never a bare yes/no: present (inside an observed membership range), uncertain (inside an unobserved bracket around a join or exit: plausible, not proven), absent (observed not a member, within coverage) or out_of_coverage (outside the index's observed history: cannot be answered). Treat uncertain and out_of_coverage as no signal; never collapse either into absent. data.matched_spell gives the certain range and the bracket edges. meta.coverage states the observed window and answerable_through, the last date the answer can be trusted for; meta.freshness.lag_days says how far the projection trails the newest snapshot. Use the index name from /v1/reference/indices (one with has_constituents: true); an index with no membership history is a 404.