[t1amat]

It seems useful to denote REPL input vs output, and to be able to identify output relating to input.

[petters]

Sure, but the [5] feels unnecessary in a finished document like this. I mean, the output should be right after the input anyway, right?

[mkl]

It's so it's easy to refer to things, like equation numbers in a maths book, or line numbers in code. E.g. people talking about someone's notebook here on HN can be specific and clear.