Function learn_provider_id 

pub fn learn_provider_id<'a>(
    all_headers: impl IntoIterator<Item = &'a RawHeaders>,
) -> Option<String>

The authserv-id your provider writes, learned from its mail: the id of the topmost Authentication-Results header of most messages.

A provider that records its checks adds its header on top of whatever the message carried, so the topmost one is its own (RFC 8601 §5). A sender can forge headers below it, never above. When the provider records nothing, the topmost header is whatever senders wrote, and no id wins a majority: then nothing is learned and trust waits for Sioul’s own checks. Sioul’s own results sit above the provider’s, under .invalid: they are skipped.