John's right that it does take more work to change the implementation, e.g. to fix errors, when it's in the SRFI document.  That's because my rule is that we have to announce changes made to the document itself, including fixes, but that fixes to the implementation (outside the document) don't require public announcement.  Also, I need to be able to reach the author, which clearly won't be a problem in the short term.  I hope it won't be a problem in the long term.

Some earlier SRFIs incorporated the implementation directly in the SRFI document.  Furthermore, your implementation is small, so the likelihood of a problem is smaller.

I'll defer to your decision on this, Lassi, but I slightly prefer to move the implementation out of the document.