PLEASE PLEASE an Undo

[luisphysics]

What do I mean? Ok for example on my Ti-inspire if I am typing an equation say
// Ok I'm gonna give a trivial example

10-1

and before pressing enter I accidently press ctrl+del
I think oh no!!!!!
Then I think ...wait TI has undo
I can press ctrl+z

and then 10-1 is back on the input screen.

It doesn't record each button press but that is fine... it is something.
...................
What about Prime? No such luck.