My paper [2], which appeared in the special issue dedicated to the late Professor Brian Hartley, contained an error in Lemma 2.2 as a result of overlooking the case that the exponents ki on p. 359 may be divisible by arbitrary powers of p, which was pointed out by Professor H. Smith. In order to overcome this situation all of Section 2, except Lemma 2.1, has been rewritten. However this does not cause any change in the rest of the paper. In particular the original assertions remain correct.