reserve m, n for Nat;

theorem
  for n, k being Nat holds k in NatDivisors n iff 0 < k & k divides n
proof
  let n, k be Nat;
  hereby
    assume k in NatDivisors n;
    then ex l being Nat st l = k & l <> 0 & l divides n;
    hence 0 < k & k divides n;
  end;
  thus thesis;
end;
