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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def Seek(self, offset, whence=os.SEEK_SET):
 """Moves the reading cursor."""


    if whence == os.SEEK_SET:
      self._offset = offset
    elif whence == os.SEEK_CUR:
      self._offset += offset
    elif whence == os.SEEK_END:
      self._offset = self._length + offset
    else:
      raise ValueError("Invalid whence argument: %s" % whence)