thanks for that additional piece. I'll put insert-<left/right> on my list of possible variants. I think we come back to naming after the initial port is done and everyone can play with it. Generally, I made the observation to better be careful with names since it's too easy to alienate other or trigger wrong assumptions.

Honestly, my knowledge of Haskell is rather limited and rusted. Hence, I am having difficulties understanding what exactly these operations with a sequence of elements. Can you give an example or some pseude/smalltalk code from your use-case and library?

Changing the subject a wee bit, there's an operation family
in my library, and I wonder how it would fit into Transducers?
To avoid bias, here's a specification in Haskell (for lists,
because I haven't had any luck installing Data.Witherable).

uccessorBy, predecessorBy :: (a -> a -> Ordering) -> a -> [a] -> a
successor, predecessor :: Ord a => a -> [a] -> a

successor = successorBy compare

successorBy cmp x = minimumBy cmp . filter (\y -> cmp x y == LT)

predecessor = predecessorBy compare

predecessorBy cmp = successorBy (flip cmp)

The reason these operations exist is to pick neighbouring
elements in SortedCollections and SortedSets. But they make
*sense* for any Enumerable. So there are "generic"
definitions with orderrides for those two classes.

A filter + a reduce . Traditionally, a #select:thenFold:ifNone:
in order to avoid building an intermediate collection. That much
I see how to do with transducers. But you can't get the desired
override for #successor:[sortBlock:][ifNone:] by overriding
#select:thenFold:ifNone: in SortedCollection or SortedSet. So what
*should* one do?