anchor.diff:      onUpdate.add(upd0);
   
       if (showNormals) {
  -      var mdNormal = glf.extractNormals(geometry.meshDef);
  +      var mdNormal = mdt.extractNormals(geometry.meshDef);
         var meshNormal = new glf.Mesh()..setData(ctx.gl, mdNormal);
         var programCtxN = glf.loadProgramContext(ctx.gl, Uri.parse("packages/glf/shaders/default.vert"), Uri.parse("packages/glf/shaders/default.frag"));
   
  
positive.code: 
  
    _add(renderer, ctx) {
      geometry.meshDef = glf.makeMeshDef_plane(dx: 3.0, dy: 3.0);
      glf.makeNormalMatrix(geometry.transforms, geometry.normalMatrix);
      var material = new Material()
      ..ctx = ctx
      ..cfg = (ctx) {
        ctx.gl.uniform3f(ctx.getUniformLocation(glf.SFNAME_COLORS), 0.0, 0.5, 0.5);
  
negative.code:     _dt  = 0.0;
    }
  }
  
  var mdt = new glf.MeshDefTools();
  
  class Main {
  
  