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  _____      ___      _____     _____     ______  
 / ____||   / _ \\   / ____||  |  ___||  /_____// 
/ //---`'  / //\ \\ / //---`'  | ||__    `____ `  
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 \_____|| |_||  |_|| \_____||  |_____||  `__ `    
  `----`  `-`   `-`   `----`   `-----`   /_//     
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發送短信 : On the diamond operator related to the wave equation