reserve n,m,k for Nat,
        p,q for n-element XFinSequence of NAT,
        i1,i2,i3,i4,i5,i6 for Element of n,
        a,b,c,d,e for Integer;

theorem Th4:
  for p be (n+k) -element XFinSequence st n<>0 & k <> 0 holds
    (p|n).i1 = p.i1
proof
  let p be (n+k) -element XFinSequence such that A1:n<>0 and A2:k<>0;
  i1 is Element of Segm(n);
  then reconsider I=i1 as Nat;
  k>0 by A2;then
  A3: len p = n+k & n+k > n+0 by CARD_1:def 7,XREAL_1:8;
  I in n by A1;
  hence thesis by A3,AFINSQ_1:53;
end;
