> Object.prototype.keys = function(filter_fn) {
Do not modify JavaScript's Object prototype if you want to work with
code other than your own. This has been pretty well discussed and
settled.http://erik.eae.net/archives/2005/06/06/22.13.54/ http://blog.metawrap.com/blog/June6thIsJavaScriptArrayAndObjectprototypeAwarenessDay.aspx
