reserve A,X,X1,X2,Y,Y1,Y2 for set, a,b,c,d,x,y,z for object;
reserve P,P1,P2,Q,R,S for Relation;

theorem
  X misses Y implies R|X|Y = {}
proof
  assume X misses Y;
  hence R|X|Y = R|({} qua set) by Th65
     .= {};
end;
