reserve X for BCI-algebra;
reserve x,y,z,u,a,b for Element of X;
reserve IT for non empty Subset of X;

theorem
  x\y=0.X implies (y\x)` = 0.X
proof
  assume x\y = 0.X;
  then (x\x)\(y\x) = 0.X by Th4;
  hence thesis by Def5;
end;
