The example is taken from "Explicit dataflow" section of the website, and copied below

function main(sources) {
    const decrement$ = sources.DOM
      .select('.decrement').events('click').map(ev => -1);

    const increment$ = sources.DOM
      .select('.increment').events('click').map(ev => +1);

    const action$ = Observable.merge(decrement$, increment$);
    const count$ = action$.startWith(0).scan((x,y) => x+y);

    const vtree$ = count$.map(count =>
      div([
        button('.decrement', 'Decrement'),
        button('.increment', 'Increment'),
        p('Counter: ' + count)
      ])
    );
    return { DOM: vtree$ };
  }