Fossil

        if(doAppend) content.push(lineNoTxt);
        else content.unshift(lineNoTxt);
        if(joinTr){
          content.push(trNext.querySelector(selector).innerHTML);
        }
        td.innerHTML = content.join('');
        if(joinTr) D.remove(joinTr);
        Diff.checkTableWidth(true);
        this.destroy();
        return this;
      }else if(this.FetchType.PrevDown===fetchType){
        /* Append context to previous TR. */
        // RHS line numbers...
        let startLnR = this.pos.prev.endRhs+1;
        lineno.length = lines.length;

        if(lines.length < (urlParam.to - urlParam.from)){
          /* No more data. */
          this.destroy();
        }else{
          this.maybeReplaceButtons();
          this.updatePosDebug();
        }
        Diff.checkTableWidth(true);
        return this;
      }else if(this.FetchType.NextUp===fetchType){
        /* Prepend content to next TR. */
        // RHS line numbers...
        if(doAppend){
          throw new Error("Internal precondition violation: doAppend is true.");
        }

           || lines.length < (urlParam.to - urlParam.from)){
          /* No more data. */
          this.destroy();
        }else{
          this.maybeReplaceButtons();
          this.updatePosDebug();
        }
        Diff.checkTableWidth(true);
        return this;
      }else{
        throw new Error("Unexpected 'fetchType' value.");
      }
    },

    /**