requirements.map(node => {
   if (typeof node === "string") {
     if (node.startsWith("subprograms")) {
       // Resolve the subprogram.
       return publishDocument(
         this.subprogramsRef.doc(node.split("/")[1])
       ).pipe(
         switchMap(result => {
           if (!result) {
             console.error("Could not resolve subprogram: " + node);
             return of({ display: "Specification error" });
           }
           return this.resolve(JSON.parse(result.requirements)).pipe(
             map(requirements => ({
               display: result.display,
               requirements
             }))
           );
         })
       );
     } else {
       return of(node);
     }
   } else if (node.requirements) {
     return this.resolve(node.requirements).pipe(
       map(requirements => ({ ...node, requirements }))
     );
   } else {
     return of(node);
   }
 })

 /**
  * Create a reference to a single document in a collection.
  * @param path
  */
 doc<T>(path: string): AngularFirestoreDocument<T> {
   return new AngularFirestoreDocument<T>(this.ref.doc(path), this.afs);
 }

 Array.from(Array(numberOfItems)).forEach((a, i) => {
   const name = randomName(firestore);
   batch.set(collectionRef.doc(name), FAKE_STOCK_DATA);
   names = [...names, name];
 });

 /**
  * Add data to a collection reference.
  *
  * Note: Data operation methods are done on the reference not the query. This means
  * when you update data it is not updating data to the window of your query unless
  * the data fits the criteria of the query.
  */
 add(data: T): Promise<DocumentReference> {
   return this.ref.add(data);
 }

 const observable = Observable.create(observer =>
   ref.onSnapshot(observer)