PVEngine creates a mask indicating values that need not be considered in the calculation:

However, that mask is not actually used anywhere today. It used to be, but it looks like it stopped being used in #84 during the switch from iterative to vectorized code.

I think the underlying concept may still be useful even in the current vectorized approach, so I think it's probably worth seeing if we can start using it again somehow, otherwise it should just be deleted IMHO.