code: function(filter) {
          var merged;
          if (filter && this.defaultFilter) {
              merged = new OpenLayers.Filter.Logical({
                  type: OpenLayers.Filter.Logical.AND,
                  filters: [this.defaultFilter, filter]
              });
          } else {
              merged = filter || this.defaultFilter || undefined;
          }
          return merged;
      }
docs: Method: mergeWithDefaultFilter
  Merge filter passed to the read method with the default one
  
  Parameters:
  filter - {<OpenLayers.Filter>}