title: BehaviourSubject's distinctUntilChanged() is not a function
body: <p>I am new to Rxjs
  I am trying understand BehaviourSubject
  below is my code</p>
  
  <pre><code>export interface State {
      items: Items[]
  }
  
  const defaultState = {
      items: []
  };
  
  const _store = new BehaviorSubject&lt;State&gt;(defaultState);
  
  @Injectable()
  export class Store {
      private _store = _store;
      changes = this._store.distinctUntilChanged()
          .do(() =&gt; console.log('changes'));
  
      setState(state: State) {
          this._store.next(state);
      }
  
      getState() : State {
          return this._store.value;
      }
  
      purge() {
          this._store.next(defaultState);
      }
  }
  </code></pre>
  
  <p>When i run my project then i get this error in my console</p>
  
  <pre><code>platform-browser.umd.js:1900 EXCEPTION: Error: Uncaught (in promise):
  EXCEPTION: Error during instantiation of Store! (StoreHelper -&gt; Store).
  ORIGINAL EXCEPTION: TypeError: this._store.distinctUntilChanged is not a function
  </code></pre>
  
  <p>Can anyone help me out. Also if I am trying to do is to create a Store for my model objects so if there is any other simpler way feel free to suggest it.</p>
  
  <p>Any help is appreciated.</p>