Thanks, Christian. Looks pretty similar to Anton's fix: I need to understand this fold biz. I have before: it's nicely discussed in HtDP. My program ran (using my simpler & less elegant fix) for over 12 hours before dying without any output. I need to improve the code. See if you have a suggestion:
I'm coding up a Sudoku technique that involves chains going from one of the 81 cells to another one, beginning & ending on different numbers. There's thousands, maybe tens of thousands of such chains. OK. There are rules to glue two chains together, and make new ones, which then go on this large list of chains. Etc. What's a good speed implementation for this?
Partly my problem is that I can't compile my program with beta 15. But I don't know if that's a gsc bug, or due to the fact that beta 17 fails `make check'. So I'd like to build beta 15 again, but I don't have the tar file anymore, and I don't know where the old betas are kept. When beta 18 comes out, perhaps this problem will be solved.