splitBy.ts

class Observable<Input> {
  // ... SNIP

  /** Create two new observables, where one contains only those inputs for which the condition
   * holds, and the other contains only those for which the condition does not hold
   */
  splitBy(condition: (_:Input)=>boolean): [Observable<Input>, Observable<Input>] {
      const yes = this.filter(condition);
      const no  = this.filter(i => ! condition(i));

      return [yes, no];
  }
  
  // ... SNIP
}

function piApproximation() {
  // ... SNIP
  const [inside, outside] = numberPairObservable.splitBy(inCircle);

  inside.subscribe( p => createDot(p, "green"), undefined);
  outside.subscribe( p => createDot(p, "red"), undefined);
  
  // ...
}