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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 correlation(self, n, k, error=False):
 r""" Calculate `\langle k \rangle_n`, the `k`-particle correlation function for `n`\ th-order anisotropy. :param int n: Anisotropy order. :param int k: Correlation order. :param bool error: Whether to calculate statistical error (for `\langle 2 \rangle` only). If true, return a tuple ``(corr, corr_error)``. """

        self._calculate_corr(n, k)
        corr_nk = self._corr[n][k]

        if error:
            self._calculate_corr_err(n, k)
            return corr_nk, self._corr_err[n][k]
        else:
            return corr_nk