How to use the @hpcc-js/ddl-shim.DDL2.isFilterActivity function in @hpcc-js/ddl-shim

readDDLViews(ddlViews: DDL2.IView[]) {
        for (const ddlView of ddlViews) {
            const element = new Element(this._ec).id(ddlView.id);
            this._ec.append(element);
            const hipiePipeline = element.hipiePipeline();
            this.readDatasourceRef(ddlView.datasource, hipiePipeline.datasource() as DSPicker, this._ec);
            for (const activity of ddlView.activities) {
                if (DDL2.isProjectActivity(activity)) {
                    const project = this.readProject(activity);
                    hipiePipeline.project(project);
                }
                if (DDL2.isFilterActivity(activity)) {
                    const filters = this.readFilters(activity, this._ec);
                    hipiePipeline.filters(filters);
                }
                if (DDL2.isGroupByActivity(activity)) {
                    const groupBy = this.readGroupBy(activity);
                    hipiePipeline.groupBy(groupBy);
                }
                if (DDL2.isSortActivity(activity)) {
                    const sort = this.readSort(activity);
                    hipiePipeline.sort(sort);
                }
                if (DDL2.isLimitActivity(activity)) {
                    const limit = this.readLimit(activity);
                    hipiePipeline.limit(limit);
                }
            }