private SortedSet<TypeElement> get(Map<TypeElement, SortedSet<TypeElement>> map, TypeElement typeElement) {
        return map.computeIfAbsent(typeElement, t ->  new TreeSet<>(comparator));
    }
private SortedSet < TypeElement > get ( Map < TypeElement , SortedSet < TypeElement > > map , TypeElement typeElement ) { return map . computeIfAbsent ( typeElement , t -> new TreeSet < > ( comparator ) ) ; }
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.