isEmpty() {

		// this is a more robust check for empty than ( volume <= 0 ) because volume can get positive with two negative axes

		return ( this.max.x < this.min.x ) || ( this.max.y < this.min.y ) || ( this.max.z < this.min.z );

	}
Returns true if this box includes zero points within its bounds.
Note that a box with equal lower and upper bounds still includes one
point, the one both bounds share.

@return {boolean} Whether this box is empty or not.