\Bibitem{Sko52} \by L.~A.~Skornyakov \paper The configuration~$D_9$ \jour Sb. Math. \yr 1952 \vol 72 \issue 1 \mathnet{http://mi.mathnet.ru//eng/sm5415} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=46660} \zmath{https://zbmath.org/?q=an:0046.38206}