Fossil

       of the diff, only between two diff chunks.
    */
    maybeReplaceButtons: function(){
      if(this.pos.next && this.pos.prev
         && (this.pos.endLhs - this.pos.startLhs <= Diff.config.chunkLoadLines)){
        D.clearElement(this.e.btnWrapper);
        D.append(this.e.btnWrapper, this.createButton(this.FetchType.FillGap));
        if( this.$fetchQueue && this.$fetchQueue.length>0 ){
          this.$fetchQueue = [this.FetchType.FillGap];
        if( this.$fetchQueue && this.$fetchQueue.length>1 ){
          this.$fetchQueue[1] = this.FetchType.FillGap;
          this.$fetchQueue.length = 2;
        }
      }
      return this;
    },

    /**
       Callack for /jchunk responses.

          up.from = this.pos.prev.endLhs + 1;
          fetchType = this.FetchType.FillGap;
        }
      }
      //console.debug("fetchChunk(",fetchType,")",up);
      fOpt.onerror = function(err){
        self.msg(true,err.message);
        self.$fetchQueue = [];
        self.$fetchQueue.length = 0;
      };
      Diff.fetchArtifactChunk(fOpt);
      return this;
    }
  };

  /**