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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 xyz(self):
 """Return all particle coordinates in this compound. Returns ------- pos : np.ndarray, shape=(n, 3), dtype=float Array with the positions of all particles. """

        if not self.children:
            pos = np.expand_dims(self._pos, axis=0)
        else:
            arr = np.fromiter(itertools.chain.from_iterable(
                particle.pos for particle in self.particles()), dtype=float)
            pos = arr.reshape((-1, 3))
        return pos