code: public Quaterniond rotationX(double angle) {
          double sin = Math.sin(angle * 0.5);
          double cos = Math.cosFromSin(sin, angle * 0.5);
          w = cos;
          x = sin;
          y = 0.0;
          z = 0.0;
          return this;
      }
docs: Set this quaternion to represent a rotation of the given radians about the x axis.
  
  @param angle
  the angle in radians to rotate about the x axis
  @return this