Chapter 1 - possible clarification (at the cost of length)

[dwanev]

There is a (possible) issue on chapter 1, in the following paragraph

Now, let's add and subtract <Equation>. This is a counterfactual outcome. It tells what would have been the outcome of the treated, had they not received the treatment.

It could be:

Now, let's add and subtract <Equation> (as this makes no mathematical difference and allows us to rearrange later). This is a counterfactual outcome. It tells what would have been the outcome of the treated, had they not received the treatment.