code: private SortedSet<TypeElement> get(Map<TypeElement, SortedSet<TypeElement>> map, TypeElement typeElement) {
          return map.computeIfAbsent(typeElement, t ->  new TreeSet<>(comparator));
      }
docs: From the map return the set of sub-classes or sub-interfaces. If set
  is null create a new one and return it.
  
  @param map The entire map.
  @param typeElement class for which the sub-class set is requested.
  @returns a list of sub classes.