 reserve W for WA-Lattice;
 reserve a,b,c for Element of W;

theorem :: 4.
  ((a "/\" c) "\/" (b "/\" c)) "\/" c = c
  proof
    (a "/\" c) "\/" c = c by LemmaX1; then
A1: a "/\" c <= c by Lemat0;
    (b "/\" c) "\/" c = c by LemmaX1; then
    b "/\" c <= c by Lemat0; then
    (a "/\" c) "\/" (b "/\" c) <= c by LatWal1, A1;
    hence thesis by Lemat0;
  end;
