Find height of a binary tree

FindTreeHeight.fs

type 'a Node =
  | Leaf of 'a
  | Group of ('a Node * 'a Node)

let findTreeHeight tree =
  let rec aux tree counter =
    match tree with
      | Leaf _ -> counter
      | Group (left, right) ->
         let leftTreeHeight = aux left (counter + 1)
         let rightTreeHeight = aux right (counter + 1)

         System.Math.Max (leftTreeHeight, rightTreeHeight)
  
  aux tree 0

(*********** Test Tree with height 3
      
                            Group
                        /           \         
                    Group           Group
                  /       \         /      \
              Leaf (1)   Group   Leaf (2)   Leaf (3)
                        /     \
                  Leaf (5)   Leaf (10)
*)
let tree =
  Group (Group (Leaf 1, Group ((Leaf 5), Leaf 10)), Group (Leaf 2, Leaf 3))
  
printfn "%d" <| findTreeHeight tree