Please use plait in Racket, my question was solved using the match function, but that doesn't work.

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 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.