theorem Th4:
  {} in F
proof
  consider A being Subset of X such that
A1: A in F by SUBSET_1:4;
  A misses A` by XBOOLE_1:79;
  then
A2: A /\ A` = {} by XBOOLE_0:def 7;
  A` in F by A1,Def1;
  hence thesis by A1,A2,FINSUB_1:def 2;
end;
