为什么Python-igraph在以下示例中将错误的方向添加到最后一个边缘？我怎么能阻止它？
from igraph import *
graph = Graph()
graph.add_vertices(10)
graph.add_edge(0,1)
graph.add_edge(1,4)
graph.add_edge(1,5)
graph.add_edge(1,3)
graph.add_edge(2,3)
graph.add_edge(4,6)
graph.add_edge(6,7)
graph.add_edge(6,8)
graph.add_edge(9,8)
graph.as_directed() # Same result with or without this line
print graph.es[8].tuple # (8, 9)