theorem Th3:
  K1 is Subfield of K2 implies for x st x in K1 holds x in K2
  proof
    assume K1 is Subfield of K2;
    then A1: the carrier of K1 c= the carrier of K2 by Def1;
    for x st x in K1 holds x in K2
    by A1;
    hence thesis;
  end;
