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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):
 """ Return a concretization of the contents of the file, as a flat bytestring. """

        size = self.state.solver.min(self._size, **kwargs)
        data = self.load(0, size)

        kwargs['cast_to'] = kwargs.get('cast_to', bytes)
        kwargs['extra_constraints'] = tuple(kwargs.get('extra_constraints', ())) + (self._size == size,)
        return self.state.solver.eval(data, **kwargs)