code: <U> MonoIgnoreThen<U> shift(Mono<U> newLast) {
          Objects.requireNonNull(newLast, "newLast");
          Publisher<?>[] a = ignore;
          int n = a.length;
          Publisher<?>[] b = new Publisher[n + 1];
          System.arraycopy(a, 0, b, 0, n);
          b[n] = last;
          
          return new MonoIgnoreThen<>(b, newLast);
      }
docs: Shifts the current last Mono into the ignore array and sets up a new last Mono instance.
  @param <U> the new last value type
  @param newLast the new last Mono instance
  @return the new operator set up