code: @SuppressWarnings("unchecked")
      public static <T> Observable<T> concatButIgnoreFirstSequence(Observable<?> o1,
              Observable<T> o2) {
          return Observable.concat((Observable<T>) o1.ignoreElements(), o2);
      }
docs: Returns the concatenation of two {@link Observable}s but the first
  sequence will be emitted in its entirety and ignored before o2 starts
  emitting.
  
  @param <T>
  the generic type of the second observable
  @param o1
  the sequence to ignore
  @param o2
  the sequence to emit after o1 ignored
  @return observable result of concatenating two observables, ignoring the
  first