code: public static <T> FlowableTransformer<T, T> insert(
              final Function<? super T, ? extends Maybe<? extends T>> valueToInsert) {
          return new FlowableTransformer<T, T>() {
  
              @Override
              public Publisher<T> apply(Flowable<T> source) {
                  return new FlowableInsertMaybe<T>(source, valueToInsert);
              }
  
          };
      }
docs: For every onNext emission from the source stream, the {@code valueToInsert}
  Maybe is subscribed to. If the Maybe emits before the next upstream emission
  then the result from the Maybe will be inserted into the stream. If the Maybe
  does not emit before the next upstream emission then it is cancelled (and no
  value is inserted).
  
  @param valueToInsert
  a Maybe is calculated from last source emission and subscribed to.
  If succeeds before next source emission then result is inserted
  into stream.
  @param <T>
  stream element type
  @return source with operator insert applied