reserve i for object, I for set,
  f for Function,
  x, x1, x2, y, A, B, X, Y, Z for ManySortedSet of I;

theorem     :: ZFMISC_1:27
  {x,y} c= {A} implies {x,y} = {A}
proof
  assume
A1: {x,y} c= {A};
  now
    let i be object;
    assume
A2: i in I;
    then {x,y}.i c= {A}.i by A1;
    then {x.i,y.i} c= {A}.i by A2,Def2;
    then
A3: {x.i,y.i} c= {A.i} by A2,Def1;
    thus {x,y}.i = {x.i,y.i} by A2,Def2
      .= {A.i} by A3,ZFMISC_1:21
      .= {A}.i by A2,Def1;
  end;
  hence thesis;
end;
