...
data class Tree(val id: String, val parentId: String?)
...
type<Tree> {
    // String - defines the key that will be sent from the [prepare] into [loader]
    // List<Tree> - defines the return type that the [loader] is required to return.
    // the loader is then required to return it in a map format like Map<String, List<Tree>>
    dataProperty<String, List<Tree>>("children") {
        // Step 2: This will only be called once.
        loader { keys: List<String> ->
            keys.map{ id -> id to listOf(Tree("SomeId", id)) }.toMap()
        }
    
        // Step 1: This will be called for each node in the list, or only once if it's not a list
        prepare { parent: Tree -> parent.id }
        
        // Step 3(Optional): This will be called as many times as the prepare did, but now holds the loaded values
        resolve { resolved: List<Tree> -> resolved } // This could maybe process something extra and then return some other type instead of List<Tree>
    }
    ...
}