code: function() {
              	if (!this.isSquare) { return null; }
              	var els = [], n = this.elements.length;
              	for (var i = 0; i < n; i++) {
              	    els.push(this.elements[i][i]);
              	}
              	return $V(els);
                  }
docs: If the matrix is square, returns the diagonal elements as a vector. Otherwise, returns null.