reserve L for Lattice,
  p,p1,q,q1,r,r1 for Element of L;
reserve x,y,z,X,Y,Z,X1,X2 for set;
reserve H,F for Filter of L;
reserve D for non empty Subset of L;
reserve D1,D2 for non empty Subset of L;
reserve I for I_Lattice,
  i,j,k for Element of I;

theorem Th28:
  i => i = Top I
proof
  now
    let j;
    i "/\" j [= i by LATTICES:6;
    then j [= i => i by Def7;
    hence j"\/"(i => i) = i => i;
  end;
  hence thesis by RLSUB_2:65;
end;
