[math-fun] golden

I believe that historically both (1+sqrt(5))/2 and its reciprocal (-1+sqrt(5))/2 have been called the golden ratio (maybe one is the golden ratio and the other is the golden section?), so it'd be handy if one of the two numbers were denoted phi and the other were denoted by tau. (Maybe someone more historically and/or notationally savvy than I could chime in here.) Jim