theorem
  i in Seg (l+1) implies i in Seg l or i = l+1
proof
A0: i is Nat by TARSKI:1;
  assume
A1: i in Seg(l+1);
  then i <= l+1 by FINSEQ_1:1;
  then 1 <= i & i <= l or i = l+1 by A1,FINSEQ_1:1,NAT_1:8;
  hence thesis by A0;
end;
