reserve L for Lattice;
reserve X,Y,Z,V for Element of L;

theorem
  X [= Z implies X \ Y [= Z
proof
  assume X [= Z;
  then
A1: X "/\" Y` [= Z "/\" Y` by LATTICES:9;
  Z "/\" Y` [= Z by LATTICES:6;
  hence thesis by A1,LATTICES:7;
end;
