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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def state_angle(ket0: State, ket1: State) -> bk.BKTensor: """The Fubini-Study angle between states. Equal to the Burrs angle for pure states. """

    return fubini_study_angle(ket0.vec, ket1.vec)