docs: def subtree_leaf_positions(subtree):
      """Return tree positions of all leaves of a subtree."""
      relative_leaf_positions = subtree.treepositions('leaves')
      subtree_root_pos = subtree.treeposition()
      absolute_leaf_positions = []
      for rel_leaf_pos in relative_leaf_positions:
          absolute_leaf_positions.append( subtree_root_pos + rel_leaf_pos)
      return absolute_leaf_positions