* <p>
     * Effective properties are computed from system, user and optionally project properties, layered with
     * defined precedence onto each other to achieve proper precedence. Precedence is defined as:
     * <ul>
     *     <li>System properties (lowest precedence)</li>
     *     <li>Project properties (optional)</li>
     *     <li>User properties (highest precedence)</li>

 * thrown away immediately once repository system was created and there is no need for more instances. If new
 * repository system instance needed, new instance of this class must be created. For proper shut down of returned
 * repository system instance(s) use {@link RepositorySystem#shutdown()} method on supplied instance(s).
 * <p>