code: def add_child(self, child, rangecheck=False):
          
          assert self.seqid == child.seqid, \
              (
                  'seqid mismatch for feature {} ({} vs {})'.format(
                      self.fid, self.seqid, child.seqid
                  )
              )
          if rangecheck is True:
              assert self._strand == child._strand, \
                  ('child of feature {} has a different strand'.format(self.fid))
              assert self._range.contains(child._range), \
                  (
                      'child of feature {} is not contained within its span '
                      '({}-{})'.format(self.fid, child.start, child.end)
                  )
          if self.children is None:
              self.children = list()
          self.children.append(child)
          self.children.sort()
docs: Add a child feature to this feature.