reserve X,Y,Z for set,
        x,y,z for object,
        A,B,C for Ordinal;
reserve U for Grothendieck;

theorem Th13:
  X in Y in U implies X in U
proof
  assume X in Y in U;
  then union Y in U & X c= union Y by Def2,ZFMISC_1:74;
  hence thesis by CLASSES1:def 1;
end;
