code: private mpz_t getPeer(BigInteger value, mpz_t sharedPeer) {
      if (value instanceof GmpInteger) {
        return ((GmpInteger) value).getPeer();
      }
      mpzImport(sharedPeer, value.signum(), value.abs().toByteArray());
      return sharedPeer;
    }
docs: If {@code value} is a {@link GmpInteger}, return its peer. Otherwise, import {@code value} into
  {@code sharedPeer} and return {@code sharedPeer}.