Referencer

[HaraldZealot via Digitalmars-d]

All ranges in Phobos pass by value, but if I have output range 
with state like cumulative statistics this is useless.

After discussion with Dicebot I try this work-arround:
http://dpaste.dzfl.pl/8af8eb8d0007
It is unfinished. But direction is observable.

Is this good solution? And how about to include something like 
this tool in Phobos?