(*Basically a Tree has a boolean, and a list of (char, tree(childs)). Image for easier understanding http://i.imgur.com/rHKQDyl.png

The tree contains words. let's say we have 'h'(false)->'e'(false)->'l'(false)->'l'(true)->'o'(true)
False means it's not a word, True means it's a word (in that example we have Hell and Hello).*)

type tree =  Node of bool ref  * (char * tree) list ref (* This cannot be changed *)

let empty () = Node(ref false,ref[])

(* Function make_branch: char list -> tree
    Where the char list is a word
*)
let rec make_branch lc =
   (*I must implement code here *)