code: ///////////////////////////////////////////////////////////////////////////
  //                   __                _      _   ________               //
  //                  / /   ____  ____ _(_)____/ | / / ____/               //
  //                 / /   / __ \/ __ `/ / ___/  |/ / / __                 //
  //                / /___/ /_/ / /_/ / / /__/ /|  / /_/ /                 //
  //               /_____/\____/\__, /_/\___/_/ |_/\____/                  //
  //                           /____/                                      //
  //                                                                       //
  //               The Next Generation Logic Library                       //
  //                                                                       //
  ///////////////////////////////////////////////////////////////////////////
  //                                                                       //
  //  Copyright 2015-20xx Christoph Zengler                                //
  //                                                                       //
  //  Licensed under the Apache License, Version 2.0 (the "License");      //
  //  you may not use this file except in compliance with the License.     //
  //  You may obtain a copy of the License at                              //
  //                                                                       //
  //  http://www.apache.org/licenses/LICENSE-2.0                           //
  //                                                                       //
  //  Unless required by applicable law or agreed to in writing, software  //
  //  distributed under the License is distributed on an "AS IS" BASIS,    //
  //  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or      //
  //  implied.  See the License for the specific language governing        //
  //  permissions and limitations under the License.                       //
  //                                                                       //
  ///////////////////////////////////////////////////////////////////////////
  
  package org.logicng.handlers;
  
  /**
   * Interface for a handler.  A handler can be used as callback for different time-intensive computations in order
   * to abort these computations.  There are same often used default handlers already implemented and users can
   * implement their own handlers by implementing the respective interfaces.
   * @version 2.1.0
   * @since 1.6.2
   */
  public interface Handler {
  
      /**
       * Returns whether the computation was aborted by the handler.
       * @return {@code true} if the computation was aborted by the handler, otherwise {@code false}
       */
      default boolean aborted() {
          return false;
      }
  
      /**
       * This method is called when the computation starts.
       */
      default void started() {
  
      }
  
      /**
       * Returns {@code true} if the handler is not {@code null} and is already aborted.
       * @param handler the handler to check
       * @return {@code true} if the handler is not {@code null} and is already aborted, otherwise {@code false}
       */
      static boolean aborted(final Handler handler) {
          return handler != null && handler.aborted();
      }
  
      /**
       * Null-safe helper method to start a handler.
       * @param handler the handler to start, may be {@code null}
       */
      static void start(final Handler handler) {
          if (handler != null) {
              handler.started();
          }
      }
  }
  