\Bibitem{Ers87} \by Yu.~L.~Ershov \paper Generability of admissible sets \jour Algebra Logika \yr 1987 \vol 26 \issue 5 \pages 577--596 \mathnet{http://mi.mathnet.ru/al1994} \mathscinet{https://mathscinet.ams.org/mathscinet-getitem?mr=985839}