Thank you; it's simply a do-nothing block named $nl (= newline). I found it to be useful for dividing code into some kind of paragraphs, for readability purposes. 
DISTRIBUTION OF reports the number of copies of a data element, the whole of each data element is considered key; both my GROUP BY function, and the similar block from the FDA library, operate on key-value pairs, considering key only item 1 of each data element, and grouping all data elements with the same key.
This dichotomy is completely different from the one between reference (memory position) and content (data).
Then why not offer two separate (and well-documented) DISTRIBUTION OF functions: one superfast but with limited functionality (to be applied primarily on lists with atomic elements), the other for general use but slower? Plus some way to indicate case (in-) sensitivity with the second version.
(the same reasoning as applies on the current superfast implementation of REMOVE DUPLICATES) 
True. I chose not to sort, so as to keep the solution as general as possible (sorting is not trivial for (sub-)lists, command blocks, and some more). However that may be, now that I've found the GROUP BY block in the FDA library, I've concluded my own version to be redundant.