reserve n,k,b for Nat, i for Integer;

theorem Th1:
  for f being non empty XFinSequence holds f|1 = <% f.0 %>
  proof
    let f be non empty XFinSequence;
    A1: 0 in Segm(0+1) by NAT_1:45;
    len f >= 1 by NAT_1:14; then
    len (f|1) = 1 & (f|1).0=f.0 by AFINSQ_1:54,FUNCT_1:49,A1;
    hence thesis by AFINSQ_1:34;
  end;
