The types in pairing::bls12_381 were moved into the separate bls12_381 crate when pairing 0.17.0 was released, last August. They are no longer included in the pairing crate. Also, pairing::bls12_381::Fr was renamed to bls12_381::Scalar.
I'm not sure what you're trying to access by typing a_prior. This name doesn't seem to appear in the docs for either the old or current versions of these crates.
As before, if you get an error, please include the text of the error.