theorem Th17:
  for Q being Subset of A holds w in right-Lang(Q) iff w-succ_of (
  Q, A) meets (the FinalS of A)
proof
  let Q be Subset of A;
  thus w in right-Lang(Q) implies w-succ_of (Q, A) meets (the FinalS of A)
  proof
    assume w in right-Lang(Q);
    then ex w9 st w = w9 & w9-succ_of (Q, A) meets (the FinalS of A);
    hence thesis;
  end;
  thus thesis;
end;
