Please answer promptly and correctly, and if possible, comment what the code is doing. Use the header that I provide below the question. Thank you!

let rec contains_var (e:expression) : bool =

Write a function contains_var : expression -> bool that returns true if and only if an expression contains a variable. Your implementation may be recursive (it need not be tail-recursive) or may be non-recursive and use fold_expr (even if you didn't do that task; we'll test your implementation with a working version of fold_expr.)