reserve n for Nat,
  k for Integer;

theorem Th14:
  for p,q,r being FinSequence holds
  len (p ^ q ^ r) = (len p) + (len q) + (len r)
proof
  let p,q,r be FinSequence;
  thus len (p ^ q ^ r) = (len (p ^ q)) + (len r) by FINSEQ_1:22
    .= ((len p) + (len q)) + (len r) by FINSEQ_1:22;
end;
