def _collapse(items):

    '''Collapses a subtree if the "value" element in it is NULL'''

    items = dict(items)

    try: require = cast(strings.ustr, items['require'])

    except KeyError, e: raise SyntaxError(e)

    value = items.get('value', None)

    required_node = xpath.get_1(value, require, allow_rooted=False)

    if required_node == None or xml_dom.is_empty(required_node): return None

    else: return value

funcs['_collapse'] = _collapse