/**
 * <p>Marker Interface: Contract<br/>
 * Contracts are interface-like constraints on a class that must
 * be implemented by programming conventions since the Java language
 * provides no mechanism to declare these constraints. By convention,
 * classes may publish their "pledges" to abide by the contract conventions
 * by implementing the marker interface associated with that contract.
 * These marker interfaces all extend this root marker interface.
 * While the code may or may not actually use the marker interfaces
 * at runtime, this marker interface mechanism provides as a side effect
 * a very good mechanism to document the contracts via JavaDoc in that
 * it allows both contract definition AND easy reference from each class
 * to these contract definitions because JavaDoc produces links to all
 * app-level interfaces implemented in the class's JavaDoc page.
 */
public interface Contract {}