test_Pfset_nilStream {
		var x = 0, y = 0, oe = (), ie = (), clup = EventStreamCleanup.new;
		oe = Pfset({ x = 1 }, p{}, { y = 2 }).asStream.next(ie);
		this.assert(x == 1, "Pfset on nil stream should still call the initializer function");
		this.assert(y == 2, "Pfset on nil stream should still call the cleanup function");
		this.assert(oe.isNil, "Pfset on nil stream should return nil");
		this.assert(ie.size.even, "Pfset on nil stream should add an even number of items to the input event");
		// ie.size is 2 if Pfset adds 'addToCleanup' and 'removeFromCleanup' in sequence (presently);
		// ie.size could be 0 if this add-remove pair is "optimized out" in the future.
		// The present implementation of EventStreamCleanup.update first does the add(s) then the remove(s),
		// so it "cancels out" matched add-remove pairs in the same event. But let's check that too...
		clup.update(ie);
		this.assert(clup.functions.size == 0, "Pfset on nil stream should have no effect on a cleanup-functions set");
	}

func	
Use environment variable syntax (e.g., ~x = 0) to store values in the internal environment. These values are copied into the event prototype before running the supplied pattern.


cleanupFunc
Optional. A function to evaluate when the pattern is stopped, or when the supplied pattern runs out of values. For example,