"The second trait that gives a language power is letting you muck around inside it as deeply as possible to make it do your bidding. That way, even if the designers of the language never conceived of what you’re trying to do, you can make your own changes to the language until it does exactly what you need to solve your problems elegantly. This trait is much more difficult to provide in a language"
Where Does Lisp Get Its Power? I’ve said that Lisp is a particularly powerful language. So what were the key insights that John McCarthy (and the other, later innovators … – Selection from Land of Lisp [Book]