code: def get_tree_type(tree):
      
      tree_type = tree.label()
      assert tree_type in SUBTREE_TYPES, "tree_type: {}".format(tree_type)
      return tree_type
docs: returns the type of the (sub)tree: Root, Nucleus or Satellite
  
      Parameters
      ----------
      tree : nltk.tree.ParentedTree
          a tree representing a rhetorical structure (or a part of it)