Interesting. Not exactly what I described though (sorry I did not explain very well.) What I meant, you type 2 calculations:

  > 1+1
  2
  > 10+10
  20

Then if you tap the first line ("1+1") my ideal REPL would add it to the bottom:

  > 1+1
  2
  > 10+10
  20
  > 1+1_ (with the cursor here ready to edit it)

Then you can for examlpe change a 1 to 2, then type ENTER:

  > 1+1
  2
  > 10+10
  20
  > 1+2
  3

That way you can easily compute a few things, then compare both the expressions and results of the last few calculations, and quickly make edits/corrections as needed.