code: public Set<P> pseudoSet(E el)
      {
          HashMap<P, D> map = pseudoMaps.get(el);
          if (map == null)
              return Collections.emptySet();
          else
              return map.keySet();
      }
docs: Gets all the pseudo elements that are available for the given element.
  @param el The given element
  @return A set of all pseudo elements available for the element