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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(cyg_arch='x86_64', mirrors=None):
    '''
    Update all packages.

    cyg_arch : x86_64
        Specify the cygwin architecture update
        Current options are x86 and x86_64

    CLI Example:

    .. code-block:: bash

        salt '*' cyg.update
        salt '*' cyg.update dos2unix mirrors="[{'http://mirror': 'http://url/to/public/key}]"
    '''
    args = []
    args.append('--upgrade-also')

    # Can't update something that isn't installed
    if not _check_cygwin_installed(cyg_arch):
        LOG.debug('Cygwin (%s) not installed, could not update', cyg_arch)
        return False

    return _run_silent_cygwin(cyg_arch=cyg_arch, args=args, mirrors=mirrors)