[Haskell-beginners] search a tree

Hi All, I have an expression which contains several different let statements let x = 4*5 and let Y = 6+4 for example. What I want to do is traverse the tree and just return the variables. In other words I just want it to return a list of all the variables in a tree. I have a function for Var String. So would it be something like given a String -> [String] or is this expression ->String (gives back a string) Whats the best way to check or compare? Jon