docs: 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