code: public ArrayList<String> collections() {
          string_vector v = ti.collections();
          int size = (int) v.size();
  
          ArrayList<String> l = new ArrayList<>(size);
          for (int i = 0; i < size; i++) {
              l.add(v.get(i));
          }
  
          return l;
      }
docs: This function is related to BEP38_ (mutable torrents). The
  vector returned from this correspond to the "collections" keys
  in the .torrent file. The collections from within the info-dict
  and from outside of it are included.
  <p>
  BEP38: http://www.bittorrent.org/beps/bep_0038.html
  
  @return