M-x up, or, GNU Emacs, GNU Readline... coincidence? I think not.

[Joe Corneli]

I don't know when I first realized this, but it was sometime this
semester.

And even if I'm the last person on earth to know that if you type

 M-x <whatever>

and then 

 M-x <up>

that your mini-buffer will fill up with whatever you typed... well,
I'm still very happy to know this fact.

What is also cool is that this generalizes. (So in particular, it
makes finding files with long filenames a breeze.)

This is such a great feature that I think it should maybe be
documented in the TUTORIAL.

(I don't know where else it might be documented at present, and if
its someplace obvious, sorry about raising this racket, but anyway,
I think it might also be good to document it in the

 "Repeating commands/Is there an equivalent to the `.' (dot) command
 of vi?"

node in the Emacs FAQ.)