eFVertices() {
  const vertexData = new Float32Array([
    // left column
    0, 0,
    30, 0,
    0, 150,
    30, 150,

    // top rung
    30, 0,
    100, 0,
    30, 30,
    100, 30,

    // middle rung
    30, 60,
    70, 60,
    30, 90,
    70, 90,
  ]);

  const indexData = new Uint32Array([
    0,  1,  2,    2,  1,  3,  // left column
    4,  5,  6,    6,  5,  7,  // top run
    8,  9, 10,   10,  9, 11,  // middle run
  ]);

  return {
    vertexData,
    indexData,
    numVertices: indexData.length,
