theorem Th20:
  rng(P*R) c= rng R
proof
  let y be object;
  assume y in rng(P*R);
  then consider x being object such that
A1: [x,y] in P*R by XTUPLE_0:def 13;
  ex z st [x,z] in P & [z,y] in R by A1,Def6;
  hence thesis by XTUPLE_0:def 13;
end;
