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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 zero_crossing_last(frames):
 """Finds the last zero crossing in frames"""

    frames = N.array(frames)

    crossings = N.where(N.diff(N.sign(frames)))
    # crossings = N.where(frames[:n] * frames[1:n + 1] < 0)

    if len(crossings[0]) == 0:
        print "No zero crossing"
        return len(frames) - 1
    return crossings[0][-1]