This question is related to the Answer
eval, print, showstat
Suppose one had 2 levels of evaluation and you only wanted to go back 1 level to uncover it's original components. That would be no problem if nothing was cleared, but very unfortunate if it did.
In this case, it is somewhat easy to see if the screen was erased to recover that c was equal to a+b indirectly.
It would actually be nice if we could call c and somehow get the underlying equation a+b