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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 _check_user(user, group):
    '''
    Checks if the named user and group are present on the minion
    '''
    err = ''
    if user:
        uid = __salt__['file.user_to_uid'](user)
        if uid == '':
            err += 'User {0} is not available '.format(user)
    if group:
        gid = __salt__['file.group_to_gid'](group)
        if gid == '':
            err += 'Group {0} is not available'.format(group)
    return err