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 org.logicng.datastructures.Substitution;
  
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  import java.util.stream.Stream;
  
  /**
   * Boolean negation.
   * @version 2.2.0
   * @since 1.0
   */
  public final class Not extends Formula {
  
      private final Formula operand;
      private volatile int hashCode;
  
      /**
       * Constructor.
       * @param operand the operand of the negation
       * @param f       the factory which created this instance
       */
      Not(final Formula operand, final FormulaFactory f) {
          super(FType.NOT, f);
          this.operand = operand;
          this.hashCode = 0;
      }
  
      /**
       * Returns the operand of this negation.
       * @return the operand of this negation
       */
      public Formula operand() {
          return this.operand;
      }
  
      @Override
      public int numberOfOperands() {
          return 1;
      }
  
      @Override
      public boolean isConstantFormula() {
          return false;
      }
  
      @Override
      public boolean isAtomicFormula() {
          return false;
      }
  
      @Override
      public boolean containsVariable(final Variable variable) {
          return this.operand.containsVariable(variable);
      }
  
      @Override
      public boolean evaluate(final Assignment assignment) {
          return !this.operand.evaluate(assignment);
      }
  
      @Override
      public Formula restrict(final Assignment assignment) {
          return this.f.not(this.operand.restrict(assignment));
      }
  
      @Override
      public boolean containsNode(final Formula formula) {
          return this == formula || this.equals(formula) || this.operand.containsNode(formula);
      }
  
      @Override
      public Formula substitute(final Substitution substitution) {
          return this.f.not(this.operand.substitute(substitution));
      }
  
      @Override
      public Formula negate() {
          return this.operand;
      }
  
      @Override
      public int hashCode() {
          if (this.hashCode == 0) {
              this.hashCode = 29 * this.operand.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 Not) {
              final Not otherNot = (Not) other;
              return this.operand.equals(otherNot.operand);
          }
          return false;
      }
  
      @Override
      public Iterator<Formula> iterator() {
          return new Iterator<Formula>() {
              private boolean iterated;
  
              @Override
              public boolean hasNext() {
                  return !this.iterated;
              }
  
              @Override
              public Formula next() {
                  if (!this.iterated) {
                      this.iterated = true;
                      return Not.this.operand;
                  }
                  throw new NoSuchElementException();
              }
  
              @Override
              public void remove() {
                  throw new UnsupportedOperationException();
              }
          };
      }
  
      @Override
      public Stream<Formula> stream() {
          return Stream.of(this.operand);
      }
  }
  