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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 p0(self):
 """A dictionary of the initial position of the walkers. This is set by using ``set_p0``. If not set yet, a ``ValueError`` is raised when the attribute is accessed. """

        if self._p0 is None:
            raise ValueError("initial positions not set; run set_p0")
        # convert to dict
        p0 = {param: self._p0[..., k]
              for (k, param) in enumerate(self.sampling_params)}
        return p0