Function usable 

fn usable(cert: &Cert) -> bool

Whether a certificate may serve at all: valid under Sequoia’s policy and not revoked as a whole. Sequoia’s revoked(false) on a key asks that key’s own revocation, never the certificate’s: a certificate revoked as compromised still has live, unrevoked subkeys (review of 19 October 2026, N3). A revoked certificate never takes a message or a seal and never signs; it still opens what was sent to it before.