Apply ``self.span_tokenize()`` to each element of ``strings``.  I.e.:
            return iter((self.span_tokenize(s) for s in strings))
        :rtype: iter(list(tuple(int, int)))

def span_tokenize_sents(self, strings):
        """
        Apply ``self.span_tokenize()`` to each element of ``strings``.  I.e.:
            return iter((self.span_tokenize(s) for s in strings))
        :rtype: iter(list(tuple(int, int)))
        """
        raise NotImplementedError("span_tokenizer and span_tokenzie_sents not yet implemented. ;)")
        for s in strings:
            yield list(self.span_tokenize(s))