Please use Ocaml to code instead of Pathon/C/C++/Java...

(** Question 1 ) (* TODO: Add test cases. *) let collect_variables_tests : (formula * Variable_set.t) list = [] (* TODO: Implement the function. *) let collect_variables (formula : formula) : Variable_set.t = raise Not_implemented Question 1 (10 points) Implement the function collect_variables : formula Variable_set.t. This function takes a formula and returns the set of variable names which appear anywhere in the formula. You may use any functions you want and whichever recursive style you prefer