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

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發送短信 : Local structure theory and the Ershov hierarchy