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.util;
  
  import java.util.Objects;
  
  /**
   * Data structure for a pair.
   * @param <A> the type parameter of the first entry
   * @param <B> the type parameter of the second entry
   * @version 1.2
   * @since 1.0
   */
  public class Pair<A, B> {
  
      protected final A a;
      protected final B b;
  
      /**
       * Constructs a new pair.
       * @param a the first entry
       * @param b the second entry
       */
      public Pair(final A a, final B b) {
          this.a = a;
          this.b = b;
      }
  
      /**
       * Returns the first entry of this pair.
       * @return the first entry
       */
      public A first() {
          return this.a;
      }
  
      /**
       * Returns the second entry of this pair.
       * @return the second entry
       */
      public B second() {
          return this.b;
      }
  
      @Override
      public int hashCode() {
          return Objects.hash(this.a, this.b);
      }
  
      @Override
      public boolean equals(final Object other) {
          if (this == other) {
              return true;
          }
          if (other instanceof Pair) {
              final Pair<?, ?> o = (Pair<?, ?>) other;
              return Objects.equals(this.b, o.b) && Objects.equals(this.a, o.a);
          }
          return false;
      }
  
      @Override
      public String toString() {
          return String.format("<%s, %s>", this.a, this.b);
      }
  }
  