[math-fun] Knuth circle product [was ... Zeckendorf representation]

CONJECTURE: x o y = 3 x y - x [(y+1)/tau^2] - y [(x+1)/tau^2] . PROOF: Presently skimped. WANTED: x o y o z = symmetric function of x,y,z. NOTES: x o y == x (*) y previously; tau == (1 + sqrt(5))/2; [...] == floor, integer part. WFL