I want to use the swap rate helper to build yield curve. The product may consist of a fixed leg and a floating leg. For the floating leg, we need to fix the float rate one day before the accrual start date. But when I use the swap rate helper, I have seen that it uses the MakeVanillaSwap and then VanillaSwap classes, where I cannot define the fixing days for the floating leg. But in the iborLeg, we can choose a fixing day for the product, and the default will be the fixing day of the iborIndex. So how can I solve this problem and can I still use the swap rate helper to build the yield curve? Or have I misunderstood any part of the code?