anchor.diff:    }
   
     void _initShaders() {
  -    // vertex shader source code. uPosition is our variable that we'll
  -    // use to create animation
  +    // vertex shader
       String vsSource = """
       attribute vec3 aVertexPosition;
       attribute vec3 aVertexNormal;
  
positive.code:     uniform mat3 uNMatrix;
      
      void main(void) {
        vPosition = uMVMatrix * vec4(aVertexPosition, 1.0);
        gl_Position = uPMatrix * vPosition;
        vTextureCoord = aTextureCoord;
  
negative.code: 
      varying vec4 vPosition;
      varying vec3 vNormal;
      varying vec3 vVertexNormal;
      varying vec3 vVertexPosition;
      varying vec2 vTextureCoord;
    
      uniform mat4 uMVMatrix;
  