def _is_span_inside(self, span, covered_spans):
        """Returns True if `span` falls within `covered_spans`.

        :param span: start and end indices of a span
        :type span: 2-`tuple` of `int`
        :param covered_spans: list of start and end indices for parts
                              of the text already covered by a sequence
        :type covered_spans: `list` of  2-`tuple` of `int`
        :rtype: `bool`

        """
        start = span[0]
        end = span[1]
        for c_start, c_end in covered_spans:
            if start >= c_start and end <= c_end:
                return True
        return False
Returns True if `span` falls within `covered_spans`.

        :param span: start and end indices of a span
        :type span: 2-`tuple` of `int`
        :param covered_spans: list of start and end indices for parts
                              of the text already covered by a sequence
        :type covered_spans: `list` of  2-`tuple` of `int`
        :rtype: `bool`