Polyraptor 1 etb, triggers Forerunner, Polyraptor 1 triggers.
Polyraptor 2 etb, triggers Forerunner, Polyraptor 1 and 2 trigger.
Polyraptor 3 and 4 etb, both trigger Forerunner.
First Forerunner trigger resolves, make 4 more raptors and Forerunner dies.
Second Forerunner trigger resolves, make 8...