\Bibitem{Ers90} \by Yu.~L.~Ershov \paper Forcing in admissible sets \jour Algebra Logika \yr 1990 \vol 29 \issue 6 \pages 648--658 \mathnet{http://mi.mathnet.ru/al2127} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=1294234}