reserve x,x1,x2,y,y9,y1,y2,z,z1,z2 for object,P,X,X1,X2,Y,Y1,Y2,V,Z for set;

theorem Th22:
  for f being Function holds <:f,X,Y:> c= f
proof
  let f be Function;
  (Y|`f|X) c= (Y|`f) & (Y|`f) c= f by RELAT_1:59,86;
  hence thesis;
end;
