sequences: make concat-as inline and don't special-case empty.
sequences: make concat-as inline and don't special-case empty.

dch/factor,factor/factor,bpollack/factor,bjourne/factor,slavapestov/factor,bpollack/factor,nicolas-p/factor,slavapestov/factor,AlexIljin/factor,mrjbq7/factor,slavapestov/factor,nicolas-p/factor,bjourne/factor,bpollack/factor,mrjbq7/factor,nicolas-p/factor,tgunr/factor,factor/factor,mrjbq7/factor,bjourne/factor,dch/factor,AlexIljin/factor,bjourne/factor,slavapestov/factor,dch/factor,nicolas-p/factor,slavapestov/factor,factor/factor,mrjbq7/factor,tgunr/factor,AlexIljin/factor,AlexIljin/factor,tgunr/factor,slavapestov/factor,factor/factor,bpollack/factor,nicolas-p/factor,AlexIljin/factor,bjourne/factor,nicolas-p/factor,nicolas-p/factor,AlexIljin/factor,tgunr/factor,AlexIljin/factor,tgunr/factor,dch/factor,factor/factor,bpollack/factor,bjourne/factor,dch/factor,mrjbq7/factor,dch/factor,tgunr/factor,factor/factor,bpollack/factor,bpollack/factor,mrjbq7/factor,bjourne/factor,slavapestov/factor