add tile_url option

src/tile_layer.ml

@@ -1,8 +1,13 @@
 type t = Jv.t
 
-let create_osm () =
+let create_osm ?tile_url () =
+  (* see https://wiki.openstreetmap.org/wiki/Tile_servers *)
+  let tile_url =
+    Option.fold ~some:Fun.id
+      ~none:"https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png" tile_url
+  in
   Jv.call Global.leaflet "tileLayer"
-    [| Jv.of_string "https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png"
+    [| Jv.of_string tile_url
      ; Jv.obj
          [| ( "attribution"
             , Jv.of_string

src/tile_layer.mli

@@ -1,5 +1,5 @@
 type t
 
-val create_osm : unit -> t
+val create_osm : ?tile_url:string -> unit -> t
 
 val add_to : t -> Map.t -> unit