<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_roots(self):
 """get the roots of a graph. must be a directed graph :returns: root list of nodes :rtype: Node[] """

    if self.__directionless:
      sys.stderr.write("ERROR: can't get roots of an undirected graph\n")
      sys.exit()
    outputids = self.__nodes.keys()
    #print outputids
    rootset  = set(outputids) -  set(self.__child_to_parent.keys())
    return [self.__nodes[x] for x in rootset]