title: How do I represent subflows in Python Luigi task manager?
body: <p>Suppose I have flow A->B.  Users of this flow require B.  Suppose I have a task C which requires B but I want D to happen before flow A->B.  How do I do this in <a href="http://luigi.readthedocs.io/en/stable/index.html" rel="nofollow noreferrer">Luigi</a>?</p>