private void addNormal(Vector3f normalToAdd, Map<Vector3f, Vector3f> normalMap, boolean smooth, Vector3f... vertices) {
        for (Vector3f v : vertices) {
            Vector3f n = normalMap.get(v);
            if (!smooth || n == null) {
                normalMap.put(v, normalToAdd.clone());
            } else {
                n.addLocal(normalToAdd).normalizeLocal();
            }
        }
    }
This method adds a normal to a normal's map. This map is used to merge
normals of a vector that should be rendered smooth.

@param normalToAdd
           a normal to be added
@param normalMap
           merges normals of faces that will be rendered smooth; the key is the vertex and the value - its normal vector
@param smooth the variable that indicates whether to merge normals
    (creating the smooth mesh) or not
@param vertices
           a list of vertices read from the blender file
jme3-core/src/main/java/com/jme3/scene/shape/Surface.java