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

__    __    ______     _____    _____     _____   
\ \\ / //  /_   _//   / ___//  |  ___||  /  ___|| 
 \ \/ //    -| ||-    \___ \\  | ||__   | // __   
  \  //     _| ||_    /    //  | ||__   | \\_\ || 
   \//     /_____//  /____//   |_____||  \____//  
    `      `-----`  `-----`    `-----`    `---`