code: private Func1<ArrayList<SoundCloudComment>, ArrayList<SoundCloudComment>> cacheTrackComments() {
          return new Func1<ArrayList<SoundCloudComment>, ArrayList<SoundCloudComment>>() {
              @Override
              public ArrayList<SoundCloudComment> call(ArrayList<SoundCloudComment> trackComments) {
                  if (trackComments.size() > 0) {
                      mCacheRam.tracksComments.put(trackComments.get(0).getTrackId(), trackComments);
                  }
                  return trackComments;
              }
          };
      }
docs: "Cache" the comments linked to a track retrieved from network in RAM
  to avoid requesting SoundCloud API for next call.
  
  @return {@link rx.functions.Func1} used to save the retrieved comments list