code: public synchronized Set<String> getVersions( String groupId, String artifactId )
      {
          Map<String, Map<String, Map<Artifact, Content>>> artifactMap = contents.get( groupId );
          Map<String, Map<Artifact, Content>> versionMap = ( artifactMap == null ? null : artifactMap.get( artifactId ) );
          return new TreeSet<String>( versionMap == null ? Collections.<String>emptySet() : versionMap.keySet() );
      }
docs: {@inheritDoc}