reserve x,y for object,X for set,
  f for Function,
  R,S for Relation;

theorem Th6:
  for S being natural-valued Relation st R c= S holds R is natural-valued
proof
  let S be natural-valued Relation;
  assume R c= S;
  then
A1: rng R c= rng S by RELAT_1:11;
  rng S c= NAT by Def4;
  hence rng R c= NAT by A1;
end;
