Function tag_lead_mask 

pub open spec fn tag_lead_mask(tag: Tag) -> Asn1TagLeadMask

{
    let bit = tag_lead_bit(tag);
    match tag.class {
        Class::Universal => {
            Asn1TagLeadMask {
                universal: bit,
                application: 0,
                context_specific: 0,
                private: 0,
            }
        }
        Class::Application => {
            Asn1TagLeadMask {
                universal: 0,
                application: bit,
                context_specific: 0,
                private: 0,
            }
        }
        Class::ContextSpecific => {
            Asn1TagLeadMask {
                universal: 0,
                application: 0,
                context_specific: bit,
                private: 0,
            }
        }
        Class::Private => {
            Asn1TagLeadMask {
                universal: 0,
                application: 0,
                context_specific: 0,
                private: bit,
            }
        }
    }
}