I have created a video to explain (via an example) the ReplaceOne and ReplaceLast operators in DIVEX and Composition Language 1.

ReplaceOne can be used to fix the value of one element in an array parameter by allowing us to provide a function that returns a value for one element. The resulting function will still contain the array parameter but it will represent the remainder of the array. ReplaceLast also fixes the value of one element in an array parameter (by allowing us to provide a function that returns a value) but it also removes the array parameter from the resulting function, basically closing the array parameter.

You can try the example in the browser: https://divex.dev/try-in-the-browser-cscompose/?sample=ReplaceOneReplaceLast