I've done/undone this a couple times already, but after reading this again, let me know if this is correct @caotto 

a) `Stream([KeyPointDetector(SIFT),(AggregateFrames(2)+OpticalFlow+ROI+HoGDescriptor)
/(ROI+HoGDescriptor),Cat])+BoVW`
b) `Stream([KeyPointDetector(SIFT),AggregateFrames(2),(OpticalFlow+ROI+HoGDescriptor)/(First+ROI+HoGDescriptor),Cat])+BoVW`

Both a & b are technically correct w/r/t grammar, but b might be faster?