docs: public synchronized Set<String> getGroupIds( String parentGroupId )
      {
          String path = parentGroupId.replace( '.', '/' );
          Map<String, Artifact> artifactMapper = this.children.get( path );
          if ( artifactMapper == null )
          {
              return Collections.emptySet();
          }
          Set<String> result = new HashSet<String>();
          for ( Map.Entry<String, Artifact> e : artifactMapper.entrySet() )
          {
              if ( e.getValue() == null )
              {
                  result.add( e.getKey() );
              }
          }
          return result;
      }