xPy_kl = sum_i (x_ik / X_k)(x_il / t_i) (exact: without replacement), and with a distance matrix Morgan's distance-decay interaction with kernel K_ij = t_j exp(-beta d_ij) / sum_j t_j exp(-beta d_ij).
xPy_kl = sum_i (x_ik / X_k)(x_il / t_i) (exact: without replacement), and with a distance matrix Morgan's distance-decay interaction with kernel K_ij = t_j exp(-beta d_ij) / sum_j t_j exp(-beta d_ij).