code: function(at, lines, height) {
        this.height += height;
        this.lines = this.lines.slice(0, at).concat(lines).concat(this.lines.slice(at));
        for (var i = 0; i < lines.length; ++i) lines[i].parent = this;
      }
docs: Insert the given array of lines at offset 'at', count them as having the given height.