theorem     :: ZFMISC_1:31
  union { x } = x
proof
  now
    let i be object;
    assume
A1: i in I;
    hence (union { x }).i = union ({x}.i) by MBOOLEAN:def 2
      .= union {x.i} by A1,Def1
      .= x.i by ZFMISC_1:25;
  end;
  hence thesis;
end;
