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.
