
theorem
  'not' {{}} = {{}}
proof
  union 'not' {{}} = union {{}} by Th66
    .= {} by ZFMISC_1:25;
  hence 'not' {{}} c= {{}} by ZFMISC_1:1,82;
  {} in 'not' {{}} by COH_SP:1;
  hence thesis by ZFMISC_1:31;
end;
