> append can take an improper list at the final argument (both R7RS and
 > SRFI-1 explicitly allows it).  For valid indexes, `(take xs i)` and
 > `(drop-right xs i)` never return improper lists, so I think both
 > examples are valid.

I see, I am incorrect there.

The take-right and drop-right example still uses `take` and `drop`
instead of `take-right` and `take-drop`, so it should be modified to

     (append (drop-right flist i) (take-right flist i)) = flist