theorem LemmaSum:
  x [= x "\/" y
  proof
    set v1 = x "\/" y;
A1: x "/\" v1 = x by LATTICES:def 9;
    ((x"/\"v1)"\/"(x"/\"v1))"\/"v1 = v1 by DefW33;
    hence thesis by LATTICES:def 3,A1;
  end; :: LATTICES:5 but without associativity
