The library has long had %%interval-foldl as an internal routine.

After the recent rewrite, %%interval-foldl was used to implement
array-foldl, array-reduce, array-for-each, and interval-for-each, and a
new %%interval-foldr was written to support array-foldr.

After seeing how useful %%interval-foldl and %%interval-foldr proved to
be in processing data in order over intervals and arrays, I decided to
publicize these routines as interval-foldl and interval-foldr.

It's not quite parallel to the for/* syntax that Racket has for loops,
but it can be used quite similarly.

Brad