grouping.extras: much faster group-map.
grouping.extras: much faster group-map.

factor/factor,factor/factor,mrjbq7/factor,mrjbq7/factor,tgunr/factor,mrjbq7/factor,bpollack/factor,mrjbq7/factor,tgunr/factor,factor/factor,mrjbq7/factor,bpollack/factor,mrjbq7/factor,tgunr/factor,bpollack/factor,bpollack/factor,tgunr/factor,factor/factor,tgunr/factor,bpollack/factor,factor/factor,tgunr/factor,factor/factor,bpollack/factor,bpollack/factor