function merge(geometry, object) {
    if (geometry.xParts === undefined)
        geometry.xParts = new Array();

    // Id of the next push'd element is the current
    // length of the array.
    var id = geometry.xParts.length;

    // Storing informations about the mesh, and its
    // offset inside the object.
    geometry.xParts.push({
        vertices: [geometry.vertices.length, object.vertices.length],
        faces:    [geometry.faces.length,    object.vertices.length],
        uvs:      [geometry.uvs.length,      object.vertices.length]
    });

    // Now that everything has been saved, we can make
    // the merge.
    THREE.GeometryUtils.merge(geometry, object);

    return id;
}

function remove(geometry, id) {
    if (geometry.xParts === undefined)
        return ;
    if (geometry.xParts.length >= id)
        return ;
    if (geometry.xParts[id] === null)
        return ;

    // We must not be able to remove same mesh twice
    // But we can't just splice it, or all others ids will
    // be invalids.
    var part = geometry.xParts[id];
    geometry.xParts[id] = null;

    // Removing mesh informations
    geometry.vertices.slice(part.vertices[0], part.vertices[1]);
    geometry.faces.slice(part.faces[0], part.faces[1]);
    geometry.uvs.slice(part.uvs[0], part.uvs[1]);

    // Updating index of next geometry parts
    for (var t = id; t < geometry.xParts.length; ++t) {
        var temp = geometry.xParts[t];
        temp.vertices[0] -= part.vertices[1];
        temp.faces[0] -= part.faces[1];
        temp.uvs[0] -= part.uvs[1];
    }
}