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

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')