
theorem XFF:
  for D be set, f be XFinSequence of D, g be FinSequence of D holds
  f^g = f^(FS2XFS g)
  proof
    let D be set, f be XFinSequence of D, g be FinSequence of D;
    f^g = f^(XFS2FS(FS2XFS g))
    .= f^(FS2XFS g) by XFX;
    hence thesis;
  end;
