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

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Text this: Normalisers of subgroups of the modular group