I don't know...
http://www.rosettacode.org/wiki/Mutual_recursion#Ruby
I find almost every other implementation better on the eyes; picolisp, pari/gp, nim and some others I would call elegant; the Ruby version looks like overly noisy and fairly ugly to me. Not so much (but still) the function defs, but p (Array.new(20) {|n| F(n) }) I find pretty nasty for some reason (again, not sure why).
Randomly picked example.