chore: refactor dijkstra implementation

test/dijkstra-ring/p.ml

-(* Time-stamp: <modified the 05/03/2020 (at 21:31) by Erwan Jahier> *)
+(* Time-stamp: <modified the 23/06/2020 (at 13:57) by Erwan Jahier> *)
 
 open Algo
 
@@ -12,12 +12,12 @@ let (init_state: int -> string -> 's) =
 
 let (enable_f: 's -> 's neighbor list -> action list) =
   fun e nl ->
-    let pred = List.hd nl in
+    let pred = match nl with [n] -> n | _ -> assert false in
     if e <> state pred then ["T"] else []
   
 let (step_f : 's -> 's neighbor list -> action -> 's) =
   fun e nl a ->
-    let pred = List.hd nl in
+    let pred = match nl with [n] -> n | _ -> assert false in
     match a with 
     | _ -> state pred