
theorem Th18:
  for f being Function st f is union-distributive holds f.{} = {}
proof
  let f be Function such that
A1: for A being Subset of dom f st union A in dom f holds f.union A =
  union (f.:A);
A2: {} c= dom f & f.:{} = {};
  not {} in dom f implies f.{} = {} by FUNCT_1:def 2;
  hence thesis by A1,A2,ZFMISC_1:2;
end;
