docs: function(datum) {
        var startPosition = this.beginPositionCb_(datum);
        var endPosition = this.endPositionCb_(datum);
  
        var node = new IntervalTreeNode(datum,
                                        startPosition, endPosition);
        this.size_++;
  
        this.root_ = this.insertNode_(this.root_, node);
        this.root_.colour = Colour.BLACK;
        return datum;
      }