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