Programming Forums - ocaml pattern matching

Kind of confused for some reason... give the following...

type expr = Plus of expr * expr        (* means a + b *)

          | Minus of expr * expr       (* means a - b *)

          | Times of expr * expr       (* means a * b *)

          | Divide of expr * expr      (* means a / b *)

          | Value of string            (* "x", "y", "n", etc. *)

          ;;





let rec to_string e =

  match e with

    Plus (left, right)   -> "(" ^ (to_string left) ^ " + " ^ (to_string right) ^ ")"

  | Minus (left, right)  -> "(" ^ (to_string left) ^ " - " ^ (to_string right) ^ ")"

  | Times (left, right)  -> "(" ^ (to_string left) ^ " * " ^ (to_string right) ^ ")"

  | Divide (left, right) -> "(" ^ (to_string left) ^ " / " ^ (to_string right) ^ ")"

  | Value v -> v

  ;;



let print_expr e =

  print_endline (to_string e);;

the matching in to_string is where I'm getting confused a tad
So given this :
Times (Value "n", Plus (Value "x", Value "y"))

Okay while looking it at I think I figured it out... When that goes through the matching I was getting confused on why it called to_string again with each parameter after it found a match but I guess it does it because the parameter can be folded such as the Plus and needs to be recycled to the function.
Right?