kem-with-encap {
    {field . {map}}
    {field a {str} >0} {# sntrup761-x25519-hkdf-blake2b}
                       {# mceliece6960119-x25519-hkdf-shake256}
    {field cek {bin} >0} {# wrapped CEK}
    {field encap {bin} >0}
    {field to {with fpr} optional} {# recipient's public key}
    {field from {with fpr} optional} {# sender's public key}
}
