theorem Th6:
  for A,B being Subset of X holds A in F & B in F implies A \ B in F
proof
  let A,B be Subset of X;
  assume
A1: A in F;
  assume B in F;
  then B` in F by Def1;
  then A /\ B` in F by A1,FINSUB_1:def 2;
  hence thesis by SUBSET_1:13;
end;
