Sorry for the last minutes comment.

(or any other value that is invalid for cursors and indexes), instead of making it an

error.

That's because currently, if you want to know (string-cursor-forward s c nchars)

is safe, you need to check (<= nchars (string-cursor-diff s c (string-cursor-end s))).

This is more cumbersome than index, with which you can use integer comparison in

various ways---e.g. you can recurse with "next index" as argument, and just

stop when (>= next-index end-index).  With cursors you can't pass such

"cursor beyond the end" so you have to do the range check before advancing

the cursor and recurse.

(Think about the task like "search a string for character X, if found, cut the

substring until that character, then skip N characters and make it a new 

start position and recurse")

Additionally, I noticed that there's no mention that #f can't be a valid cursor

value in the current draft.  Cursors should be disjoint to #f, for string-contains requires so.