
theorem Th14:
  for f being XFinSequence st len f >= 2 holds f|2 = <%f.0,f.1%>
proof
  let f be XFinSequence;
  assume A1: len f >= 2;
  then
A2: len(f|2) = 2 by AFINSQ_1:54;
  0 in Segm 2 by NAT_1:44; then
A3: (f|2).0 = f.0 by A1,AFINSQ_1:53;
  1 in Segm 2 by NAT_1:44; then
A4: (f|2).1 = f.1 by A1,AFINSQ_1:53;
  thus thesis by AFINSQ_1:38,A2,A3,A4;
end;
