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.Substitution;
  
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  import java.util.stream.Stream;
  
  /**
   * Super class for Boolean binary operators.
   * @version 2.2.0
   * @since 1.0
   */
  public abstract class BinaryOperator extends Formula {
  
      protected final Formula left;
      protected final Formula right;
      protected volatile int hashCode;
  
      /**
       * Constructor.
       * @param type  the type of the formula
       * @param left  the left-hand side operand
       * @param right the right-hand side operand
       * @param f     the factory which created this instance
       */
      BinaryOperator(final FType type, final Formula left, final Formula right, final FormulaFactory f) {
          super(type, f);
          this.left = left;
          this.right = right;
          this.hashCode = 0;
      }
  
      /**
       * Returns the left-hand side operator.
       * @return the left-hand side operator
       */
      public Formula left() {
          return this.left;
      }
  
      /**
       * Returns the right-hand side operator.
       * @return the right-hand side operator
       */
      public Formula right() {
          return this.right;
      }
  
      @Override
      public int numberOfOperands() {
          return 2;
      }
  
      @Override
      public boolean isConstantFormula() {
          return false;
      }
  
      @Override
      public boolean isAtomicFormula() {
          return false;
      }
  
      @Override
      public boolean containsVariable(final Variable variable) {
          return this.left.containsVariable(variable) || this.right.containsVariable(variable);
      }
  
      @Override
      public boolean containsNode(final Formula formula) {
          return this == formula || this.equals(formula) || this.left.containsNode(formula) || this.right.containsNode(formula);
      }
  
      @Override
      public Formula substitute(final Substitution substitution) {
          return this.f.binaryOperator(this.type, this.left.substitute(substitution), this.right.substitute(substitution));
      }
  
      @Override
      public Formula negate() {
          return this.f.not(this);
      }
  
      @Override
      public Iterator<Formula> iterator() {
          return new Iterator<Formula>() {
              private int count;
  
              @Override
              public boolean hasNext() {
                  return this.count < 2;
              }
  
              @Override
              public Formula next() {
                  if (this.count == 0) {
                      this.count++;
                      return BinaryOperator.this.left;
                  } else if (this.count == 1) {
                      this.count++;
                      return BinaryOperator.this.right;
                  }
                  throw new NoSuchElementException();
              }
  
              @Override
              public void remove() {
                  throw new UnsupportedOperationException();
              }
          };
      }
  
      @Override
      public Stream<Formula> stream() {
          return Stream.of(this.left, this.right);
      }
  }
  