`setdefault`, or some superior functionality as part of `transform()`

Consider the following code:

class ApplicationState(PClass):
    shared_resources = pset_field(Injectable)
    # ...

class ExtractedState(PClass):
    applications = pmap_field(str, ApplicationState)

def extract(tfstate):
    result = {}

    for mod in tfstate['modules']:
        for injectable in _extract_resources_from_module(mod):
            app_state = result.setdefault(injectable.app,
                                          {"shared_resources": set(),
                                           "service_resources": {}})
            app_state["shared_resources"].add(injectable)

   return ExtractedState.create(freeze({"applications": result}))

Constructing the pyrsistent objects is quite annoying, which is why I'm doing it with mutable objects, due to lack of setdefault or "create a default object in transform()". Ideally I'd do:

extracted_state = ExtractedState()
extracted_state = extracted_state.transform(
    ["applications", "newapp", "shared_resources"], lambda ps: ps.add(obj))

but since newapp doesn't exist it blows up...