... the interpreter already knows that _2 is the second anonymous parameter to the function literal, without parsing the "2" token.
So you could instead have:
[+ _ _ _]
... and specify the index only if you actually want to use the parameter outside of the function literal (and there, _ would still be shorthand for _1).
And BTW, this whole thing should work with rest parameters, so that in:
I'm just saying that if you have several anonymous parameters in the parameter list of the function literal, it is just as well that you represent them with the same token _within the parameter list_. The parameter list is ordered, so the interpreter knows which is which anyway. If you then want to refer to one of the anonymous parameters _outside of the parameter list_, you use a token WITH an index, to identify the position of the particular anonymous parameter in the parameter list.
