neil1234

if the actual prepayments are below the collar, then the principal paid to the support tranche is directed to the PAC. this means that more of support bond is outstanding (or the holders of the support bond have an extension risk)

I doubt they will ask us to compute the WAL for a PAC.. you will need to compute the principal paid down every month using the 2 different psa's and pick the one with the lower payment.