wiki.cyclopsgroup.org

I don't have an official document yet. Will create one.
In short, the format is either of following three:

For Collection or array, it's [ ele1, ele2, …], the expression of element follows same rules
For Map, it's { key1 = value1, key2 = value2, …}, the expression of key and value follows same rules
For others, it's what toString() returns.