<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def append_domain():
    '''
    Return append_domain if set
    '''

    grain = {}

    if salt.utils.platform.is_proxy():
        return grain

    if 'append_domain' in __opts__:
        grain['append_domain'] = __opts__['append_domain']
    return grain