[Viliam1234]

> most probably in a large code base, you'll en up with 'unless', 'if-not' and depending on how creative others are probably also 'negat-if'.

In Clojure, "if-not" is part of clojure.core (the equivalent of java.lang in Java), so a decent programmer would almost certainly be familiar with it; and if not, it would be pointed out at a code review.

People use Lisp for a while, so the obvious simple ideas were probably already noticed and implemented.

With less frequent things, implementing the same thing twice under two different names is a problem that happens regardless of the language.