Document the following piece of code:
    def step(x):
        return (x >= 0) * 1.0

    def low_tonnage(a, b):
        def low_tonnage_(tonnage):
            score = ((tonnage - b)/(a - b)) * (step(tonnage - a) - step(tonnage - b)) + 1 - step(tonnage - a)
            return score

        return low_tonnage_
 The letter w should appear at least 41 times.
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