import 'rxjs/add/observable/empty'
import 'rxjs/add/observable/of'
import 'rxjs/add/observable/fromPromise'
import 'rxjs/add/observable/concat'
import 'rxjs/add/operator/switchMap'
import 'rxjs/add/operator/map'
import 'rxjs/add/operator/mergeMap'
import 'rxjs/add/operator/takeWhile'
import 'rxjs/add/operator/catch'
import 'rxjs/add/operator/concat'
import 'rxjs/add/operator/filter'