code: public static <T> List<T> concat(List<T> l, T t) {
          return concat(l, singletonList(t));
      }
docs: Concatenates (appends) a single elements to an existing list
  
  @param l   the list onto which to append the element
  @param t   the element to append
  @param <T> the type of elements of the list
  
  @return a <strong>new</strong> list componsed of the first list elements and the new element