[ 200 Service home ]
+ [ 210 Credentials ]
    + [ 211 Username ]
    + [ 212 Password ]
    + [ 213 API Key ]
    + [ 214 --> Join ]
    + [ 215 --> documentation ]
+ [ 220 Accept-Language ]

Example language codes or make up your own!
    + [ 221 MRU list (maybe later) ]
    + [ 222 Whole list ]
source