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

theorem :: 4.  :: in LATWAL_1 appears [= instead of <=
  ((a "\/" c) "/\" (b "\/" c)) "/\" c = c
  proof
A2: a "\/" c >= c by LatWal2;
    (b "\/" c) "/\" c = c by LemmaX2; then
    b "\/" c >= c by Lemat01; then
    (a "\/" c) "/\" (b "\/" c) >= c by A2,LatWal3;
    hence thesis by Lemat01;
  end;
