reserve X,Y,z,s for set, L,L1,L2,A,B for List of X, x for Element of X,
  O,O1,O2,O3 for Operation of X, a,b,y for Element of X, n,m for Nat;

theorem
  NOT NOT NOT O = NOT O
  proof
    thus NOT NOT NOT O = id(X\dom NOT NOT O) by Th37
    .= id(X\dom O) by Th38 .= NOT O by Th37;
end;
