EOPL - 2.3.3 - Exercise 2.20

It's simple to add new operations when the data is represented as an abstract syntax tree. Here's the operation has_association for stacks as defined in the previous exercise:

// Exercise 2.20
let rec has_association env sym = 
    match env with
      EmptyEnv -> false
    | ExtendedEnv (syms, vals, env') -> (List.mem sym syms) || 
                                        (has_association env' sym)