import com.google.common.base.Objects;
import java.io.Serializable;
import javax.annotation.CheckForNull;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * An ordering that orders elements by applying an order to the result of a function on those
 * elements.
 */
@GwtCompatible(serializable = true)
@ElementTypesAreNonnullByDefault

  /**
   * Returns the {@link Executor} that will be used to run this service. Subclasses may override
   * this method to use a custom {@link Executor}, which may configure its worker thread with a
   * specific name, thread group or priority. The returned executor's {@link
   * Executor#execute(Runnable) execute()} method is called when this service is started and
   * stopped, and should return promptly.

  protected void triggerShutdown() {}

  /**
   * Returns the {@link Executor} that will be used to run this service. Subclasses may override
   * this method to use a custom {@link Executor}, which may configure its worker thread with a
   * specific name, thread group or priority. The returned executor's {@link
   * Executor#execute(Runnable) execute()} method is called when this service is started, and should
   * return promptly.
   *

 * user-defined properties.
 *
 * <p>A {@code Graph} built by this class has the following default properties:
 *
 * <ul>
 *   <li>does not allow self-loops
 *   <li>orders {@link Graph#nodes()} in the order in which the elements were added (insertion
 *       order)
 * </ul>
 *
 * <p>{@code Graph}s built by this class also guarantee that each collection-returning accessor