/*        Main functionality
   Procedure CancelBookOnHold processes the request to cancel a book with 
   specified Call Nuber, which has been on hold for a patron with given User ID.
*/
Procedure CANCEL_BOOK_ON_HOLD
{
   while (1)
   
     switch (kind_of_button_pressed) 
       {

      case CLEAR:
        clear the contents in the entry field;
      case HLEP:
        open new pop-up window displaying help;
      case CANCEL:
        close the Hold a Book window and return control to the Book Administration Window;
      case SEARCH:    /* search the User Id */
        search for the patron ID by poping up the Searching Customer window;
      case LIST:
        display a list of all the books on hold by the customer with the given Customer ID.
      case PRINT:
        prints a list of books being held by this patron.
      case OK button:
        if (syntax for User ID is not correct)
           display error message about the User ID;
        else if (syntax for Call Number is not correct)
           display error message about the Call Number;
        else if (User Id does not exist in database)
           display non-existent input error message;
        else if (Call_Number does not exist in database)
           display non-existent input error message;
        else 
          {
          retrieve the customer's record from the database;
          if the customer dosn't have this on on hold
             display message("Cancel a hold was unsuccessful: The patron doesn't have this book on hold.");
          else
             {
              update the customer record;
              retrieve the book's record and change the book's circulation_status to 'available';
              display message( "Cancel a hold is successful");
             }
          } 
      }
}