system.rs

let storage = () // this is the placeholder I'm using for your component storage;

//
// this following stuff would go in your system init
//

// create bitsets for each kind in your enum, keep in system struct data
let (head, body, turn, tail) = 
	(BitSet::default(), BitSet::default(), BitSet::default(), BitSet::default());

// get a flagged storage reader id to also keep in your system struct data
let event_reader_id = storage.register_reader();

//
// and then this following stuff would go in your system's run function
// which would need access to Entities as well as your storage, ofc
//

// get the unread events for your storage
let events = storage.channel().read(event_reader_id);

for event in events {
	match event {
		ComponentEvent::Modified(id) => { 
			let component = storage.get(entities.entity(*id)).unwrap();
			
			// probably a better way to do this lol
			// but basically this entity might be in one of the wrong bitsets at this point
			// but we don't know which! we just know which one it belongs in now
			// so we add it to that.
			head.remove(*id);
			body.remove(*id);
			turn.remove(*id);
			tail.remove(*id);
			
			match component {
				head => head.add(*id),
				// etc
			};
		},
		ComponentEvent::Inserted(id) => { 
			let component = storage.get(entities.entity(*id)).unwrap();
			
			match component {
				head => head.add(*id),
				// etc
			};
		},
		ComponentEvent::Removed(id) => {
			// same problem as before! 
			// but not that big of a deal I guess
			head.remove(*id);
			body.remove(*id);
			turn.remove(*id);
			tail.remove(*id);
		},
	};
}

for (component, _) in (storage, head).join() {
	// now do this to only get heads
	// and change the bitset to choose which enum branch
}