code: function() {
              var u = new CSG.Vector3D(this.elements[0], this.elements[4], this.elements[8]);
              var v = new CSG.Vector3D(this.elements[1], this.elements[5], this.elements[9]);
              var w = new CSG.Vector3D(this.elements[2], this.elements[6], this.elements[10]);
  
              // for a true orthogonal, non-mirrored base, u.cross(v) == w
              // If they have an opposite direction then we are mirroring
              var mirrorvalue = u.cross(v).dot(w);
              var ismirror = (mirrorvalue < 0);
              return ismirror;
          }
docs: determine whether this matrix is a mirroring transformation