code: def get_nucsat_subtrees(parented_tree):
      
      if is_leaf(parented_tree):
          return [parented_tree]
      
      nucsat_children = []
      for child in parented_tree:
          if  is_leaf(child) or child.label() in ('N', 'S'):
              nucsat_children.append(child)
          else:
              nucsat_children.extend( get_nucsat_subtrees(child) )
      return nucsat_children
docs: Return all direct children of the given tree, that are either
      a nucleus, satellite or a leaf node (i.e. all children except
      for relation nodes.)