Mine

def sumtree(tree):
    if tree == []:
        return 0
    elif isinstance(tree[0], list):
        return sumtree(tree[0]) + sumtree(tree[1:])
    else:
        return tree[0] + sumtree(tree[1:])

(define (sumtree tree)
  (cond ((null? tree) 0)
        ((pair? (car tree)) (+ (sumtree (car tree))
                               (sumtree (cdr tree))))
        (else (+ (car tree) (sumtree (cdr tree))))))