public Quaternion addLocal(Quaternion q) {
        this.x += q.x;
        this.y += q.y;
        this.z += q.z;
        this.w += q.w;
        return this;
    }
Adds the argument and returns the (modified) current instance.

<p>Seldom used. To combine rotations, use
{@link #multLocal(com.jme3.math.Quaternion)} or
{@link #mult(com.jme3.math.Quaternion, com.jme3.math.Quaternion)}
instead of this method.

@param q the quaternion to add (not null, unaffected unless it's
    {@code this})
@return the (modified) current instance (for chaining)
jme3-core/src/main/java/com/jme3/math/Quaternion.java