Function extract_family 

pub fn extract_family(
    schema_version: impl Into<String>,
    stream_mode: StreamMode,
    ast: &Ast,
    visibility_key: impl Into<String>,
) -> Result<Option<FamilyExtraction>, QueryKeyError>

Extract ast’s family, if it can join one (module docs). Ok(None) ⇒ the query cannot join any family and falls back to its QueryKey — today’s path, byte for byte. Err only on the (practically unreachable) serialization failure QueryKey shares.