theorem
  for n being natural Number holds n in Segm(n+1)
proof
  let n be natural Number;
A1: n is Nat by TARSKI:1;
  n<n+1 by XREAL_1:29;
  then n in {l where l is Nat:l<n+1} by A1;
  hence thesis by AXIOMS:4;
end;
