code: def copy_subtree(ret, element, msg):
          '''copy_subtree
  
          High-level api: Copy element as a subtree and put it as a child of ret.
  
          Parameters
          ----------
  
          element : `Element`
              A node in a model tree.
  
          msg : `str`
              Message to be added.
  
          ret : `Element`
              A node in self.tree.
  
          Returns
          -------
  
          None
              Nothing returns.
          '''
  
          sub_element = ModelDiff.process_attrib(deepcopy(element), msg)
          ret.append(sub_element)
          return sub_elemenf copy_subtree(ret, element, msg):
          '''copy_subtree
  
          High-level api: Copy element as a subtree and put it as a child of ret.
  
          Parameters
          ----------
  
          element : `Element`
              A node in a model tree.
  
          msg : `str`
              Message to be added.
  
          ret : `Element`
              A node in self.tree.
  
          Returns
          -------
  
          None
              Nothing returns.
          '''
  
          sub_element = ModelDiff.process_attrib(deepcopy(element), msg)
          ret.append(sub_element)
          return sub_element
docs: copy_subtree
  
          High-level api: Copy element as a subtree and put it as a child of ret.
  
          Parameters
          ----------
  
          element : `Element`
              A node in a model tree.
  
          msg : `str`
              Message to be added.
  
          ret : `Element`
              A node in self.tree.
  
          Returns
          -------
  
          None
              Nothing returns.