Problem 7: Label Sum (100 pts)

Using the data abstraction above, write a function that sums up the entries of a tree, assuming that the entries are all numbers.

Hint: You may want to use map with a helper function to sum list entries.

Note: Do not violate the abstraction barrier!

(define (label-sum t)
  'YOUR-CODE-HERE
)

;;; Tests
; scm> (label-sum t1)
; 36