docs: def _is_inside(self, span1, span2, covered_spans):
          """Returns True if both `span1` and `span2` fall within
          `covered_spans`.
  
          :param span1: start and end indices of a span
          :type span1: 2-`tuple` of `int`
          :param span2: start and end indices of a span
          :type span2: 2-`tuple` of `int`
          :param covered_spans: lists of start and end indices for parts
                                of the texts already covered by a sequence
          :type covered_spans: `list` of two `list`s of 2-`tuple` of `int`
          :rtype: `bool`
  
          """
          if self._is_span_inside(span1, covered_spans[0]) and \
             self._is_span_inside(span2, covered_spans[1]):
              return True
          return False