fix to adding tokens to dictionary while thresholding

        if nwords == -1:

            nwords = len(self)

        if padding_factor > 1:

            i = 0

            while nwords % padding_factor != 0:

                if nwords >= len(self):

                    self.add_symbol('madeupword{:04d}'.format(i))

                    i += 1

                nwords += 1

        new_symbols = self.symbols[:self.nspecial]

        new_count = self.count[:self.nspecial]

                new_count.append(count)

            else:

                break

        threshold_nwords = len(new_symbols)

        if padding_factor > 1:

            i = 0

            while threshold_nwords % padding_factor != 0:

                new_symbols.append('madeupword{:04d}'.format(i))

                i += 1

                threshold_nwords += 1

        assert min(new_count[self.nspecial:]) >= threshold

        assert len(new_symbols) <= nwords

        assert len(new_symbols) % padding_factor == 0

        self.count = tuple(new_count)