[This is my comment re-posted from that thread. I hate zero-based indexing.]
In this expression:
[+ _1 _2 _3]
... 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:
Using [+ _ _ _] with the same meaning as [+ _1 _2 _3] would be confusing I'd say. [+ _ _ _] should mean the same as [+ _1 _1 _1] if it is allowed at all.
(= x2 [+ _ _])
Explicit numbering will make sure the arguments stay in the correct order so my earlier remark of allowing any underscore-beginning string is probably wrong.
