Function checked_completed_share 

fn checked_completed_share<S: PixSpec>(
    msg: PixScalar<S>,
    share: &PartialSsaShare<S>,
) -> Result<DefaultShare<IdentifierPrimeField<PixScalar<S>>, IdentifierPrimeField<PixScalar<S>>>, S::Pseudonym>

Decodes a partial share and enforces the inexpensive structural checks that must precede both live-cycle and recovered-tail liveness accounting.