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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 _dist_corr(dist, phi1, phi2, phi3):
 """ Generic distance-decaying correlation function :param dist: Distance between catchment centrolds in km :type dist: float :param phi1: Decay function parameters 1 :type phi1: float :param phi2: Decay function parameters 2 :type phi2: float :param phi3: Decay function parameters 3 :type phi3: float :return: Correlation coefficient, r :rtype: float """

        return phi1 * exp(-phi2 * dist) + (1 - phi1) * exp(-phi3 * dist)