I have one more consideration, and a pair of questions, which I forgot to mention when I wrote my previous followup:

1. I don't know if it's the same reason why that particular symbol was chosen, but I find the appearance of [\ ] to be extremely representative of the "take partial results" thingie since it looks much like a triangle which is maximally evident here, given that the output "is also a triangle" but has more general sense with common reduction situations one may naturally feel psychologically at ease with, charachterized by "results that grow." (Sorry for the sloppy parliance, which I believe to be appropriate here, though.)
2. I also wonder if the same behaviour can be obtained by means of a suitable adverb (i.e. whether one such adverb is provided!) acting on [ ]... well if adverbs do apply to meta-operators at all, that is, which I ignore altogher;
3. last and perhaps most importantly, given that [\ ] returns the list of partial reductions, shouldn't it for completeness also include the 0-th one? Namely, the neutral element for the bynary operation passed to it, if any. (I already can feel that the "if any" bit could prompt for a negative answer, yet I feel that a positive one would be mathematically more appropriate.)