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]))

a) Implement a sum function that takes a tree and returns the sum of the numbers in the tree. Example: (sum (node 5 (leaf 6) (leaf 7))) should produce 18.

b) Implement the function negate, which takes a tree and returns a tree that has the same shape, but with all the numbers negated. Example: (negate (node 5 (leaf 6) (leaf 7))) should produce (node -5 (leaf -6) (leaf -7)).