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

theorem :: ZFMISC_1:76
  EmptyMS I c= A
proof
  let i be object;
  assume i in I;
  EmptyMS I.i = {} by PBOOLE:5;
  hence thesis by XBOOLE_1:2;
end;
