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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 build_prev_op(self):
 """ Compose 'list-map' which allows to jump to previous op, given offset of current op as index. """

        code = self.code
        codelen = len(code)
        # 2.x uses prev 3.x uses prev_op. Sigh
        # Until we get this sorted out.
        self.prev = self.prev_op = [0]
        for offset in self.op_range(0, codelen):
            op = code[offset]
            for _ in range(instruction_size(op, self.opc)):
                self.prev_op.append(offset)