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;
  
  import java.util.LinkedHashSet;
  
  /**
   * Boolean conjunction.
   * <p>
   * Invariants:
   * - has at least two elements
   * - does not contain duplicates
   * - does not contain complementary literals
   * - does not contain constants
   * @version 2.2.0
   * @since 1.0
   */
  public final class And extends NAryOperator {
  
      /**
       * Constructor.
       * @param operands the stream of operands
       * @param f        the factory which created this instance
       */
      And(final LinkedHashSet<? extends Formula> operands, final FormulaFactory f) {
          super(FType.AND, operands, f);
      }
  
      @Override
      public boolean evaluate(final Assignment assignment) {
          for (final Formula op : this.operands) {
              if (!op.evaluate(assignment)) {
                  return false;
              }
          }
          return true;
      }
  
      @Override
      public int hashCode() {
          return hashCode(31);
      }
  
      @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 And) { // this is not really efficient... but should not be done anyway!
              return compareOperands(((And) other).operands);
          }
          return false;
      }
  }
  