Malcev 1954: a variety is congruence-permutable iff has ternary term p(x,y,z) with p(x,x,y)=y + p(x,y,y)=x; congruence-lattice properties characterize varieties.
Malcev 1954: a variety is congruence-permutable iff has ternary term p(x,y,z) with p(x,x,y)=y + p(x,y,y)=x; congruence-lattice properties characterize varieties.