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