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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 update(backend=None):
    '''
    Update the fileserver cache. If no backend is provided, then the cache for
    all configured backends will be updated.

    backend
        Narrow fileserver backends to a subset of the enabled ones.

        .. versionchanged:: 2015.5.0
            If all passed backends start with a minus sign (``-``), then these
            backends will be excluded from the enabled backends. However, if
            there is a mix of backends with and without a minus sign (ex:
            ``backend=-roots,git``) then the ones starting with a minus
            sign will be disregarded.

            Additionally, fileserver backends can now be passed as a
            comma-separated list. In earlier versions, they needed to be passed
            as a python list (ex: ``backend="['roots', 'git']"``)

    CLI Example:

    .. code-block:: bash

        salt-run fileserver.update
        salt-run fileserver.update backend=roots,git
    '''
    fileserver = salt.fileserver.Fileserver(__opts__)
    fileserver.update(back=backend)
    return True