theorem
  for D being set, p being FinSequence of D, n being Nat
  holds (FS2XFS p)|n = FS2XFS(p|n) & (FS2XFS p)/^n = FS2XFS(p/^n)
proof
  let D be set, p be FinSequence of D, n be Nat;
  thus (FS2XFS p)|n = FS2XFS XFS2FS((FS2XFS p)|n)
    .= FS2XFS((XFS2FS FS2XFS p)|n) by Th17
    .= FS2XFS(p|n);
  thus (FS2XFS p)/^n = FS2XFS XFS2FS((FS2XFS p)/^n)
    .= FS2XFS((XFS2FS FS2XFS p)/^n) by Th17
    .= FS2XFS(p/^n);
end;
