Function signature_seam_of_route 

pub fn signature_seam_of_route(
    by: &ByClause,
    actors: &HashMap<String, ActorDecl>,
) -> Option<SignatureSeam>

#2155: the Signature seam a by clause’s route is signed with at the system tier — its one actor’s, or an actor sum’s first Signature member’s (what by Forged/by Stale sign as). None when it has none, and for a sum with an open member (sum_has_open_member), whose seam never refuses a bad signature.