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;
reserve F,F1,F2 for filtering Operation of X;

theorem
  F OR NOT F = id X
  proof
    dom(F OR NOT F) = X & F OR NOT F c= id X by Th50,Def21;
    hence thesis by SYSREL:17;
  end;
