<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def graph_key_from_tag(tag, entity_index):
 """Returns a key from a tag entity Args: tag (tag) : this is the tag selected to get the key from entity_index (int) : this is the index of the tagged entity Returns: str : String representing the key for the given tagged entity. """

    start_token = tag.get('start_token')
    entity = tag.get('entities', [])[entity_index]
    return str(start_token) + '-' + entity.get('key') + '-' + str(entity.get('confidence'))