fix tikz check

TikzEditor/MarkovChain.ml

@@ -5,6 +5,7 @@
     type attribute_id = int
     type attribute = [ `Choice of string list
                      | `ControlPoint of DrawingGeom.point
+                     | `Check of bool
                      | `String of string
                      | `Color of string ]
 

TikzEditor/TikzGraph.ml

@@ -5,6 +5,7 @@ type arc = string list * DrawingGeom.path_elem list
 type attribute_id = A | B of int | C of int | D of int
 type attribute = [ `Choice of string list
                  | `ControlPoint of DrawingGeom.point
+                 | `Check of bool
                  | `String of string
                  | `Color of string ]
 
@@ -169,6 +170,7 @@ let update_arc_attr (at,cn) attr_id = function
         D id -> Some ((replace_list id (Some ("color="^newv)) at),cn)
       | _ -> None
     end
+  | Some (`Check _) -> None
   | None ->
      let _,listat = List.fold_left (fun (i,l) nat ->
                         match attr_id with