The srfi text says the content is set to "the difference of the previous content and fx". Does it mean (- previous-content fx), (- fx previous-content), or even (abs (- fx previous-content))?

Reference implementation is (- fx previous-content) . When I read the srfi I took it as (- previous-content fx) so it was a bit of a surprise. (Maybe I'm conditioned to old CPU's SUB instructions).

The wording can be clarified, e.g. "the subtraction of previous-content from fx"?

--shiro