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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 concretize(self, **kwargs):
 """ Returns a list of the packets read or written as bytestrings. """

        lengths = [self.state.solver.eval(x[1], **kwargs) for x in self.content]
        kwargs['cast_to'] = bytes
        return [b'' if i == 0 else self.state.solver.eval(x[0][i*self.state.arch.byte_width-1:], **kwargs) for i, x in zip(lengths, self.content)]