docs: def add_child(self, child):
          """Children are GFFFeatures and are defined when added. This is done to avoid memory overheads
          that may be incurred by GFF files that have millions of rows.
  
          """
          child_id = getattr(child, 'id', None)
          if child_id:
              if not hasattr(self, 'children'):
                  self.children = {}
              if child_id not in self.children:
                  self.children[child_id] = child