code: public static <T> List<T> concat(List<T> l1, List<T> l2) {
          ArrayList<T> l = new ArrayList<>(l1);
          l.addAll(l2);
          l.trimToSize();
          return l;
      }
docs: Concatenates two lists into one
  
  @param l1  the first list to concatenate
  @param l2  the second list to concatenate
  @param <T> the type of element of the lists
  
  @return a <strong>new</strong> list composed of the two concatenated lists elements