code: function rotateX(m, angleInRadians, dst) {
    dst = dst || new MatType(16);
  
    const m10 = m[4];
    const m11 = m[5];
    const m12 = m[6];
    const m13 = m[7];
    const m20 = m[8];
    const m21 = m[9];
    const m22 = m[10];
    const m23 = m[11];
    const c = Math.cos(angleInRadians);
    const s = Math.sin(angleInRadians);
  
    dst[4]  = c * m10 + s * m20;
    dst[5]  = c * m11 + s * m21;
    dst[6]  = c * m12 + s * m22;
    dst[7]  = c * m13 + s * m23;
    dst[8]  = c * m20 - s * m10;
    dst[9]  = c * m21 - s * m11;
    dst[10] = c * m22 - s * m12;
    dst[11] = c * m23 - s * m13;
  
    if (m !== dst) {
      dst[ 0] = m[ 0];
      dst[ 1] = m[ 1];
      dst[ 2] = m[ 2];
      dst[ 3] = m[ 3];
      dst[12] = m[12];
      dst[13] = m[13];
      dst[14] = m[14];
      dst[15] = m[15];
    }
  
    return dst;
  }
docs: Modifies the given 4-by-4 matrix by a rotation around the x-axis by the given
  angle.
  @param {module:twgl/m4.Mat4} m The matrix.
  @param {number} angleInRadians The angle by which to rotate (in radians).
  @param {module:twgl/m4.Mat4} [dst] matrix to hold result. If none new one is created..
  @return {module:twgl/m4.Mat4} m once modified.
  @memberOf module:twgl/m4