'with_component' missing from System (GDscript)

[Glitshy]

I downloaded build #966 and tried to implement a new system, but the 'with_component' method doesn't exist:

extends System

func _prepare():
	with_component(ECS.TransformComponent, System.IMMUTABLE)

I get this error:

Function "with_component()" not found in base self.

The Godot editor shows me the following methods in the System class:

void _prepare() virtualvoidexecute_after(system_name: int)
voide xecute_before(system_name: int)
void execute_in(phase: Phase, dispatcher: int = 0xFFFFFFFF)
int get_system_id() const
void with_databag(databag_id: int, mutability: Mutability)
void with_events_emitter(event_id: int)
void with_events_receiver(event_id: int, emitter_name: String)
void with_query(query: Object)voidwith_storage(component_id: int)

Sorry if I am missing something extremely obvious, but it's supposed to be here, right?