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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 __do_filter_sub(self, scanline, result):
 """Sub filter."""

        ai = 0
        for i in range(self.fu, len(result)):
            x = scanline[i]
            a = scanline[ai]
            result[i] = (x - a) & 0xff
            ai += 1