function rotationX(angleInRadians, dst) {
    dst = dst || new MatType(16);

    var c = Math.cos(angleInRadians);
    var s = Math.sin(angleInRadians);

    dst[ 0] = 1;
    dst[ 1] = 0;
    dst[ 2] = 0;
    dst[ 3] = 0;
    dst[ 4] = 0;
    dst[ 5] = c;
    dst[ 6] = s;
    dst[ 7] = 0;
    dst[ 8] = 0;
    dst[ 9] = -s;
    dst[10] = c;
    dst[11] = 0;
    dst[12] = 0;
    dst[13] = 0;
    dst[14] = 0;
    dst[15] = 1;

    return dst;
  }
Creates a 4-by-4 matrix which rotates around the x-axis by the given angle.
@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} The rotation matrix.
@memberOf module:twgl/m4