reserve x, y for object, I for set,
  A, B, X, Y for ManySortedSet of I;

theorem :: ZFMISC_1:92
  X in A implies X c= union A
proof
  assume
A1: X in A;
  let i be object;
  assume
A2: i in I;
  then X.i in A.i by A1;
  then X.i c= union (A.i) by ZFMISC_1:74;
  hence thesis by A2,Def2;
end;
