發送短信 : On the semigroup whose elements are subgraphs of a complete graph

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