Please use plait in Racket

The following Tree datatype implements a binary tree with a number in each node and leaf: (define-type Tree (leaf [val : Number]) (node [val : Number] [left : Tree] [right : Tree]))

Implement the function positive-trees?, which takes a list of trees and returns #t if every member of the list is a positive tree, where a positive tree is one whose numbers sum to a positive value.

Hint 1: This function takes a list, not a tree, so dont try to use the template for a tree function.

Hint 2: Use your sum function as a helper.