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_1:93
  union { A,B } = A (\/) B
proof
  now
    let i be object;
    assume
A1: i in I;
    hence (union { A,B }).i = union ({A,B}.i) by MBOOLEAN:def 2
      .= union {A.i,B.i} by A1,Def2
      .= A.i \/ B.i by ZFMISC_1:75
      .= (A (\/) B).i by A1,PBOOLE:def 4;
  end;
  hence thesis;
end;
