def most_frequent_terms(self, count=0):
        terms = sorted(self._terms.items(), key=lambda i: -i[1])
        terms = tuple(i[0] for i in terms)
        if count == 0:
            return terms
        elif count > 0:
            return terms[:count]
        else:
            raise ValueError(
                "Only non-negative values are allowed for count of terms.")