Constant SEQUENCE_ID_UNKNOWN 

pub const SEQUENCE_ID_UNKNOWN: i64 = i64::MIN; // -9_223_372_036_854_775_808i64

The id a backend reports when it cannot supply one — the Rust spelling of the ABI’s NROS_RMW_SEQUENCE_ID_UNKNOWN (INT64_MIN). The C vtable adapter pre-sets both out-parameters to it, so a backend that never writes them reads as “no id” rather than as id 0, which is a legal id.