docs: def add_child(self, child):
          """
          Add a child to the tree. Extends discards all comments
          and whitespace Text. On non-whitespace Text, and any
          other nodes, raise a syntax error.
          """
  
          if isinstance(child, Comment):
              return
  
          # ignore Text nodes with whitespace-only content
          if isinstance(child, Text) and not child.text.strip():
              return
  
          super(Extends, self).add_child(child)