(* your code goes here *)
 
let dfs graph visited start_node = 
  let rec explore path visited node = 
    if List.mem node visited then visited else     
      let new_path = node :: path in 
      let edges    = List.assoc node graph in
      let visited  = List.fold_left (explore new_path) visited edges in
      node :: visited
  in explore [] visited start_node
 
let toposort graph = 
  List.fold_left (fun visited (node,_) -> dfs graph visited node) [] graph
 
type recipe = Eggs | Milk | Wheat | Mix | Cook | Serve
 
module Node = struct 
  type t = recipe 
end
 
let graph = [ Wheat, [Eggs;Milk;Mix] ;
              Milk,  [Mix] ;
              Eggs,  [Mix] ;
              Mix,   [Cook] ;
              Cook,  [Serve] ;
              Serve, [] ]
 
 
let sorted = toposort graph
 
 
let str_recipe = function
| Eggs -> "Eggs"
| Milk -> "Milk"
| Wheat -> "Wheat"
| Mix -> "Mix"
| Cook -> "Cook!"
| Serve -> "Serve"
 
let () = List.iter (fun i -> Printf.printf "%s -> " (str_recipe i)) sorted

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