let grwork = [("wake","shower"); ("shower","dress");
	      ("dress","go");    ("wake","eat");
	      ("eat","washup");  ("washup","go")];;

let g1 = [("wake","shower"); ("shower","wake")];;

let topsort0 graph =
  let rec nexts = function
      (x,[])       -> []
    | (x,(a,b)::l) -> if a=x then b::nexts(x,l) else nexts(x,l)
  in	
  let rec sort = function
      ([], visited)    -> visited
    | (x::xs, visited) ->
	if List.mem x visited
	then    sort(xs, visited)
	else    sort(xs, sort(nexts(x,graph), x::visited)) (* Buggy! Compare below for correct versions *)
  in	  
  let rec
      firsts = function
          []              -> []
        | (a,b)::pairlist -> a::(firsts pairlist)
  in
    sort(firsts graph, [])
;;


let topsort1 graph =
  let rec nexts = function
      (x,[])       -> []
    | (x,(a,b)::l) -> if a=x then b::nexts(x,l) else nexts(x,l)
  in	
  let rec sort = function
      ([], visited)    -> visited
    | (x::xs, visited) ->
	if List.mem x visited
	then    sort(xs, visited)
	else    sort(xs, x::sort(nexts(x,graph), visited))
  in
    sort(List.map (fun (a,b) -> a) graph, [])
;;

let topsort2 graph =
  let nexts(x,graph) =
    List.fold_right
      (@)
      (List.map (fun (a,b) -> if a=x then [b] else []) graph)
      []
  in	
  let rec sort = function
      ([], visited)    -> visited
    | (x::xs, visited) ->
	if List.mem x visited
	then    sort(xs, visited)
	else    sort(xs, x::sort(nexts(x,graph), visited))
  in
    sort(List.map (fun (a,b) -> a) graph, [])
;;