I came to the comments hoping for commiseration on how hard W is, but I suppose I can live with the reassurance that it is in fact gettable. Maybe in a few hours I'll be ready to re-enter the equation trenches and stare at it again.
oh, good lord. I wasn't expecting that gur gjb jbhyq unir qvssrerag zrnavatf!
Now I have solved the final challenge, but it did require me to rethink my understanding in one case. (That might be a nudge in the right direction. Can nudge further on request, but I bet you'll get there. :-) )
