tail call (tail recursive)

http://en.wikipedia.org/wiki/Tail_call
http://angelonotes.blogspot.com/2011/03/tail-call-tail-recursive.html
http://seedmanee.blogspot.com/2010/09/tail-recursion.html
https://medium.com/binary-dreams/a6acdd71a94d
http://www.kungfuice.com/index.php/2009/02/08/reversing-a-string-in-scala/