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 Th23:
  0.X in AtomSet(X)
proof
  for z being Element of X st z\0.X=0.X holds z=0.X by Th2;
  then 0.X is atom;
  hence thesis;
end;
