The docs describes that properties can have properties too. https://docs.janusgraph.org/advanced-topics/advschema/#multi-properties
For example:

alice -age-> 23 -source-> foo
-address-> "something" -source-> bar

My question is, how are these properties of properties serialized in GraphML and GraphSON? In JSON properties are simple key-value pairs, for example

{
"alice": {
"age": 23,
"address": "something
}
}

so I don't really understand where would the "source" properties be added. Thanks.