http://forums.msexchange.org/m_1800629635/mpage_1/key_/tm.htm
http://forums.koplayer.com/posts/t36266-that-is--the-derivative-of-the-position-with-respect-to-time
http://www.m3forum.net/m3forum/showthread.php?p=1067542865
https://www.pathofexile.com/forum/view-thread/1771376
http://carsandcoffeerichmond.com/ccr/forum/showthread.php?tid=4464