 reserve n for Nat;

theorem Th2:
  for L being non empty ZeroStr, p being Polynomial of L
  holds deg p is Element of NAT iff p <> 0_.L
proof
  let L be non empty ZeroStr;let p be Polynomial of L;
  now assume p <> 0_.L; then
      len p <> 0 by POLYNOM4:5; then
      len p + 1 > 0 + 1 by XREAL_1:6; then
      len p >= 1 by NAT_1:13;
      hence deg(p) is Element of NAT by INT_1:3;
    end;
  hence thesis by HURWITZ:20;
end;
