code: def get_node_type(tree):
      
      node_type = tree[0].label()
      assert node_type in NODE_TYPES, "node_type: {}".format(node_type)
      return node_type
docs: returns the node type (leaf or span) of a subtree (i.e. Nucleus or Satellite)
  
      Parameters
      ----------
      tree : nltk.tree.ParentedTree
          a tree representing a rhetorical structure (or a part of it)