program(id: string) {
   return new Program(
     this,
     this.ref.collection("programs").doc(id),
     this.ref.collection("subprograms")
   );
 }

 extension(id: string): Program {
   return new Program(
     this.institution,
     this.ref.collection("extensions").doc(id),
     this.subprogramsRef
   );
 }

 delete(): Promise<void> {
   return this.ref.delete();
 }

 get sequences(): Observable<string[]> {
   return publishIndex(this.ref.collection("sequences"));
 }

 sequence(id: string) {
   return new Sequence(this.ref.collection("sequences").doc(id));
 }

 course(id: string) {
   return new Course(this, this.ref.collection("courses").doc(id));
 }

 get watches(): Observable<string[]> {
   return publishIndex(this.ref.collection("watches"));
 }

 watch(id: string) {
   return new Watch(this.ref.collection("watches").doc(id));
 }

 get programs(): Observable<string[]> {
   return publishIndex(this.ref.collection("programs"));
 }

 term(id: string) {
   return new Term(this.ref.collection("terms").doc(id));
 }

 get extensions(): Observable<string[]> {
   return publishIndex(this.ref.collection("extensions"));
 }

 section(id: string) {
   return new Section(this.ref.collection("sections").doc(id));
 }

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

 /**
  * Create a reference to a sub-collection given a path and an optional query
  * function.
  * @param path
  * @param queryFn
  */
 collection<T>(path: string, queryFn?: QueryFn): AngularFirestoreCollection<T> {
   const collectionRef = this.ref.collection(path);
   const { ref, query } = associateQuery(collectionRef, queryFn);
   return new AngularFirestoreCollection<T>(ref, query, this.afs);
 }

 /**
  * Update some fields of a document without overwriting the entire document.
  * @param data
  */
 update(data: Partial<T>): Promise<void> {
   return this.ref.update(data);
 }

 /**
  * Create or overwrite a single document.
  * @param data
  * @param options
  */
 set(data: T, options?: SetOptions): Promise<void> {
   return this.ref.set(data, options);
 }