identity quasigroup 05B07 08A05; Secondary 05A15 20N15 39B52 Primary 20N05 alinear variety alinearity bunch cancellability central functional equation left linear mediality middle linearity parastrophe parastrophic orbit parastrophic primary equivalence parastrophic symmetry pseudomediality reducibility right semi-alinear semi-central semi-linear truss звідність квазігрупа