Pretty neat.

Curious if it's out of score or valuable to make the output/input behave more like a variable than copy/paste? e.g. line 1 is "2 + 3", so you click on the result, "5" and you use that in a new expression: "5 + 20". However, if you go and change the original line to "7 + 3" the second expression could update to "10 + 20"