Step 2: For i=1,...,T-1, S_{i+1}=a_i*(S_i+b_i(x_{i+1}-x_i))+(1-a_i)* {y_i}\over {I_i}. Then, I_ {i+1}={y_i}\over {S_i}. Then, b_{i+1}=d_i b_i + (1-d_i) * {S_{i+1} - S_i}\over {x_{i+1}-x_i}.
