;; weight : fern -> number
(define (weight a-fern)
  (cond
    [(leaf? a-fern) 
     (* 0.5 (leaf-area a-fern))]
    [(branch? a-fern) 
     (+ (* pi
           (branch-thickness a-fern)
           (branch-thickness a-fern)
           (branch-length a-fern))
        (weight-lof (branch-left a-fern))
        (weight-lof (branch-right a-fern)))]))
     
;; weight-lof : list-of-fern -> number
(define (weight-lof a-lof)
  (cond
   [(empty? a-lof) 0]
   [(cons? a-lof) 
    (+ (weight (first a-lof))
       (weight-lof (rest a-lof)))]))