If you want a bit of history: http://www.digitalmars.com/d/archives/digitalmars/D/announce/RFC_on_range_design_for_D2_12922.html#N12922 http://www.digitalmars.com/d/archives/digitalmars/D/announce/Revised_RFC_on_range_design_for_D2_13211.html
http://web.archive.org/web/20090112000313/http://ssli.ee.washington.edu/~aalexand/d/tmp/std_range.html That last link (although outdated) is probably more descriptive than what we have now.