In case you are new to Mathematica or don’t know it at all, here is a short description of it as advertised on Wolfram’s website. But in order to really feel the frustration the present Mathematica Undo function or rather malfunction can cause, you’ll have to try it yourself :) or check out here.

What Is Mathematica?

Almost any workflow involves computing results, and that’s what Mathematica does—from building a hedge-fund trading website or publishing interactive engineering textbooks, to developing embedded image-recognition algorithms or teaching calculus.

Mathematica is renowned as the world’s ultimate application for computations. But it’s much more—it’s the only development platform fully integrating computation into complete workflows, moving you seamlessly from initial ideas all the way to deployed individual or enterprise solutions.
Source and more: wolfram.com/mathematica