code: function(idx, vertex, opt) {
  
          var vertices = this.vertices();
  
          idx = (isFinite(idx) && idx !== null) ? (idx | 0) : 0;
          if (idx < 0) idx = vertices.length + idx;
  
          // getter
          if (arguments.length <= 1) return this.prop(['vertices', idx]);
          // setter
          return this.prop(['vertices', idx], vertex, opt);
      }
docs: Vertices API