[Users] group layer in a tree

[François Boussuges]

Hello.
I have a little problem with the layer tree:

I have three layers :
var medicago_site_co = new OpenLayers.Layer.Vector(
            "Medicago site de collecte",{
                strategies : [ new OpenLayers.Strategy.BBOX(),
                               new OpenLayers.Strategy.Fixed()
                ],
                protocol: new OpenLayers.Protocol.HTTP(
                        {url : "/geoserver/wfs?",
                            params: {format:"WFS",
                            service: "WFS",
                            version: "1.0.0",
                            request: "GetFeature",
                            typename: "MEDICAGO_SITE_COLLECTE",
                            SRS: "EPSG:4326"},
                            format : new OpenLayers.Format.GML()
                        }),

                        geometryName : "GEOMETRY",
                        visibility: false,
                        styleMap: style1

            });
    var medicago_site_or = new OpenLayers.Layer.Vector(
            "Medicago site d origine",{
                strategies : [ new OpenLayers.Strategy.BBOX(),
                               new OpenLayers.Strategy.Fixed()
                ],
                protocol: new OpenLayers.Protocol.HTTP(
                        {url : "/geoserver/wfs?",
                            params: {format:"WFS",
                            service: "WFS",
                            version: "1.0.0",
                            request: "GetFeature",
                            typename: "MEDICAGO_SITE_ORIGINE",
                            SRS: "EPSG:4326"},
                            format : new OpenLayers.Format.GML()
                        }),

                        geometryName : "GEOMETRY",
                        visibility: false,
                        styleMap: style1

            });
    var medicago_site_se = new OpenLayers.Layer.Vector(
            "Medicago site de selection",{
                strategies : [ new OpenLayers.Strategy.BBOX(),
                               new OpenLayers.Strategy.Fixed()
                ],
                protocol: new OpenLayers.Protocol.HTTP(
                        {url : "/geoserver/wfs?",
                            params: {format:"WFS",
                            service: "WFS",
                            version: "1.0.0",
                            request: "GetFeature",
                            typename: "MEDICAGO_SITE_SELECTION",
                            SRS: "EPSG:4326"},
                            format : new OpenLayers.Format.GML()
                        }),

                        geometryName : "GEOMETRY",
                        visibility: false,
                        styleMap: style1

            });

I want to group this layer in a same layer group but I don't find the 
solution, I Geoext demo the layer group is WMS layers, have you a solution?

Thank