code: function(options) {
          OpenLayers.Control.prototype.initialize.apply(this, [options]);
          
          this.registry = OpenLayers.Util.extend({
              "moveend": this.getState
          }, this.registry);
          
          var previousOptions = {
              trigger: OpenLayers.Function.bind(this.previousTrigger, this),
              displayClass: this.displayClass + " " + this.displayClass + "Previous"
          };
          OpenLayers.Util.extend(previousOptions, this.previousOptions);
          this.previous = new OpenLayers.Control.Button(previousOptions);
          
          var nextOptions = {
              trigger: OpenLayers.Function.bind(this.nextTrigger, this),
              displayClass: this.displayClass + " " + this.displayClass + "Next"
          };
          OpenLayers.Util.extend(nextOptions, this.nextOptions);
          this.next = new OpenLayers.Control.Button(nextOptions);
  
          this.clear();
      }
docs: Constructor: OpenLayers.Control.NavigationHistory
  
  Parameters:
  options - {Object} An optional object whose properties will be used
  to extend the control.