theorem Th14:
  for f being complex-valued Function holds
  f <> 0*n implies -f <> 0*n
  proof
    let f be complex-valued Function;
    assume
A1: f <> 0*n;
    assume -f = 0*n;
    then --f = -(0*n);
    hence thesis by A1,Th13;
  end;
