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.formulas;
  
  import org.logicng.datastructures.Assignment;
  
  /**
   * Boolean implication.
   * @version 2.2.0
   * @since 1.0
   */
  public final class Implication extends BinaryOperator {
  
      /**
       * Constructor.
       * @param left  the left-hand side operand
       * @param right the right-hand side operand
       * @param f     the factory which created this instance
       */
      Implication(final Formula left, final Formula right, final FormulaFactory f) {
          super(FType.IMPL, left, right, f);
      }
  
      @Override
      public boolean evaluate(final Assignment assignment) {
          return !this.left.evaluate(assignment) || this.right.evaluate(assignment);
      }
  
      @Override
      public Formula restrict(final Assignment assignment) {
          return this.f.implication(this.left.restrict(assignment), this.right.restrict(assignment));
      }
  
      @Override
      public int hashCode() {
          if (this.hashCode == 0) {
              this.hashCode = 37 * this.left.hashCode() + 39 * this.right.hashCode();
          }
          return this.hashCode;
      }
  
      @Override
      public boolean equals(final Object other) {
          if (other == this) {
              return true;
          }
          if (other instanceof Formula && this.f == ((Formula) other).f) {
              return false; // the same formula factory would have produced a == object
          }
          if (other instanceof Implication) {
              final Implication otherImp = (Implication) other;
              return this.left.equals(otherImp.left) && this.right.equals(otherImp.right);
          }
          return false;
      }
  }
  