docs: def make_span(parented_tree):
      """create a 'span' or 'leaf' subtree for dis/lisp/RST-DT-formatted trees.
      
      Examples:
             span     (a subtree that covers the leaves 1 to 7)
           ___|____   
          1        7 
  
          leaf        (a subtree that only covers leaf 7)
           |   
           7
      """
      all_leaves = all_leaf_positions(parented_tree)
      if is_root(parented_tree):
          return t('span', ['1', str(len(all_leaves))])
      
      subtree_leaves = subtree_leaf_positions(parented_tree)
      if len(subtree_leaves) == 1:
          edu_id = all_leaves.index(subtree_leaves[0]) + 1
          return t('leaf', [str(edu_id)])
      elif len(subtree_leaves) > 1:
          first_edu_id = all_leaves.index(subtree_leaves[0]) + 1
          last_edu_id = all_leaves.index(subtree_leaves[-1]) + 1
          return t('span', [str(first_edu_id), str(last_edu_id)])
      else:
          raise NotImplementedError('Subtree has no leaves')