reserve X for set;

theorem Th1:
  for X,Y being set holds union {X,Y,{}} = union {X,Y}
proof
  let X,Y be set;
  thus union {X,Y,{}} = union ({X,Y} \/ {{}}) by ENUMSET1:3
    .= union {X,Y} \/ union {{}} by ZFMISC_1:78
    .= union {X,Y} \/ {} by ZFMISC_1:25
    .= union {X,Y};
end;
