User.AppendFile: Efficiently append data to an existing file
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Implement this method:

.. function:: AppendFile(filename string, data []byte) (err error)
    
    If the file does not exist, return an error.

    If the file does exist, append the given content to the file.
    
    You are **not** required to check the integrity of the existing file before
    appending the new content (integrity verification is allowed, but not
    required). If you detect an integrity violation or the append operation
    cannot proceed for any reason, return an error.

    :param filename: File name
    :type filename: string
    :param data: Data to be appended to an existing file.
    :type data: []byte

    :rtype: error

.. warning::
    Do not forget that your design must satisfy **all** requirements, including:

    #. The implementation of ``AppendFile()`` must satisfy the efficiency
       requirement (see :ref:`requirement_efficiency`).
    
    #. Users can have multiple active user sessions at once (see
       :ref:`requirement_user_sessions`).