content: /* 
   * Kodkod -- Copyright (c) 2005-present, Emina Torlak
   *
   * Permission is hereby granted, free of charge, to any person obtaining a copy
   * of this software and associated documentation files (the "Software"), to deal
   * in the Software without restriction, including without limitation the rights
   * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
   * copies of the Software, and to permit persons to whom the Software is
   * furnished to do so, subject to the following conditions:
   *
   * The above copyright notice and this permission notice shall be included in
   * all copies or substantial portions of the Software.
   *
   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
   * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
   * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
   * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
   * THE SOFTWARE.
   */
  package kodkod.ast;
  
  import kodkod.ast.visitor.ReturnVisitor;
  import kodkod.ast.visitor.VoidVisitor;
  
  
  /**
   * A node in the abstract syntax tree (DAG).  A node
   * can accept a ReturnVisitor and have a sequence of
   * zero or more children.
   * 
   * @specfield children: int ->lone Node
   * @specfield components: set Node
   * @invariant children.Node = { i: int | 0 <= i < #children }
   * @invariant components= children[int]
   * @author Emina Torlak
   */
  public abstract class Node {
      
      /**
       * Accepts the given visitor and returns the result
       * of the visit (i.e. the result of the call visitor.visit(this))
       * @return the result of being visited by the given visitor
       * @throws NullPointerException visitor = null
       */
      public abstract <E, F, D, I> Object accept(ReturnVisitor<E, F, D, I> visitor);
     
      /**
       * Accepts the given void visitor by calling visitor.visit(this).
       * @throws NullPointerException visitor = null
       */
      public abstract void accept(VoidVisitor visitor);
  
      /**
       * Returns a string representation of this node. 
       * For a pretty-printed string, use {@linkplain kodkod.util.nodes.PrettyPrinter}.
       * @return a string representation of this node
       * @see kodkod.util.nodes.PrettyPrinter
       */
      public abstract String toString();
  }
  