reserve L for Lattice;
reserve X,Y,Z,V for Element of L;
reserve L for D_Lattice;
reserve X,Y,Z for Element of L;
reserve L for 0_Lattice;
reserve X,Y,Z for Element of L;
reserve L for B_Lattice;
reserve X,Y,Z,V for Element of L;

theorem
  (X "\/" Y)` [= X`
proof
  (X "\/" Y)` = X` "/\" Y` by LATTICES:24;
  hence thesis by LATTICES:6;
end;
