theorem Th14:
  for A, B being Subset of X holds Intersection(A followed_by B) = A /\ B
proof
  let A, B be Subset of X;
  set A1 = A followed_by B;
  Complement A1= A` followed_by B` by Lm1;
  then rng Complement A1 = {A`,B`} by FUNCT_7:126;
  then Union Complement A1 = A` \/ B` by ZFMISC_1:75;
  then (Union Complement A1)` = A`` /\ B`` by XBOOLE_1:53;
  hence thesis;
end;
