發送短信 : On the diamond operator related to the wave equation

  ______    _____     _  __     ___              
 /_   _//  |  ___||  | |/ //   / _ \\    ____    
 `-| |,-   | ||__    | ' //   | / \ ||  |    \\  
   | ||    | ||__    | . \\   | \_/ ||  | [] ||  
   |_||    |_____||  |_|\_\\   \___//   |  __//  
   `-`'    `-----`   `-` --`   `---`    |_|`-`   
                                        `-`