text: package expressiontree.commands;
  
  import expressiontree.platspecs.Platform;
  import expressiontree.tree.TreeOps;
  
  /**
   * No-op command.  This plays the role of the "ConcreteCommand" in the
   * Command pattern.
   */
  public class NullCommand 
         extends UserCommand {
      /**
       * Constructor that provides the appropriate {@code TreeOps} and
       * the requested format.
       */
      public NullCommand(TreeOps context) {
          super(context);
      }
  
      /** 
       * Set the desired format. 
       */
      public void execute() {
          // no-op.
      }
  
      /** 
       * Print the valid commands available to users. 
       */
      public void printValidCommands(boolean verboseField) {
          Platform platform = Platform.instance();
      	platform.disableAll(verboseField);
          platform.outputMenu("",
                              "",
                              "");
          platform.outputMenu("1a.",
                              "format",
                              "[post-order]");
          platform.outputMenu("1b.",
                              "set",
                              "[variable=value]");
          platform.outputMenu("2.",
                              "expr",
                              "[expression]");
          platform.outputMenu("3a.",
                              "eval",
                              "[post-order]");
          platform.outputMenu("3b.",
                              "print",
                              "[in-order | pre-order | post-order| level-order]");
          platform.outputMenu("0c.",
                              "quit",
                              "");
          platform.outputMenu("",
                              "",
                              "");
      }
  }
  