public Quaternion oppositeLocal() {
        return opposite(this);
    }
Changes the quaternion to a rotation with the same axis and the angle
increased by 180 degrees. If the quaternion isn't normalized, or if the
rotation angle is very small, the result is undefined.

@return the (modified) current instance
jme3-core/src/main/java/com/jme3/math/Quaternion.java