
theorem Th20:
  for L being non empty ZeroStr for p being Polynomial of L holds
  deg p = -1 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 POLYNOM4:3;
    hence deg p = -1;
  end;
  hence thesis by POLYNOM4:5;
end;
