`#version 300 es  
//#version directive must occur on the first line of the shader

 
const vec2 vertexPosition [4] = vec2[] (
                                          vec2(-0.5,  0.5),
                                          vec2(-0.5, -0.5),
                                          vec2( 0.5,  0.5),
                                          vec2( 0.5, -0.5)
                                        );


// all shaders have a main function
 
void main()
{
  // gl_Position is a special variable a vertex shader
  // is responsible for setting
  gl_Position = vec4( vertexPosition [ gl_VertexID ], 0.0, 1.0);

//    gl_PointSize = 10.0;
  }`