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>>> topSort $ buildDirectedGraph 4 4 $ U.fromList [(0,1),(0,2),(1,3),(2,3)]
Just [0,1,2,3]
>>> topSort $ buildDirectedGraph 2 0 U.empty
Just [0,1]
>>> topSort $ buildDirectedGraph 2 2 $ U.fromList [(0,1),(1,0)]
Nothing