Q:

Prototype and YUI

I saw a reference on a company intranet recently that "You should use YUI instead of prototype". (formerly Prototype was company recommended JS framework).
Is that because YUI builds on/extends Prototype? 
Or merely implements a super-set of its functionality?

A:

YUI is a completely separate library from to Prototype. YUI has more built in UI elements than Prototype and a very different design philosophy.

