You could try something like this, with a concept:
concept S1 {
o String[] csv
}
asset S2 identified by sID {
o String sID
o S1 [] s1
}
Example data would look like:
{
"$class": "org.acme.model.S2",
"sID": "SR26",
"s1": [{"csv":["a","b"]},{"csv":["c","d"]}]
}