StringTake
 Usage
• StringTake["string", n] 返回由"string"中前 n个字符构成的字符串.
• StringTake["string", -n] 返回由"string"中后 n个字符构成的字符串.
• StringTake["string",  n ] 返回 "string"中第 n个字符. • StringTake["string",  m, n ] 给出 "string"中从 m到 n位置的字符构成的字符串.
Notes
• StringTake 使用标准的 序列指定 (参见 A.3.5节). • 例如: StringTake["abcdefg", 3]  . • StringTake["string",  m, n, s ] 以步长 s返回从 m到 n位置的字符.
 Further Examples
This takes  characters starting from the end of the string.
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This takes the character at position 5.
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This takes the characters  through  .
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This takes characters  through  in steps of  .
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