Charles Choi wanted my input on what to do for such an attack. Therefore you have ot get out of the choke. I showed this combination attack from a standing position. Thanks to Roy Gutierrez for being my uke and Robert Farmer for taking the pictures.
Attacker sets right forearm choke.
Attacker brings knife up to you with his left hand. Your left hand immediately comes up to block. You don't know if it's to stab you or threaten you. Block his knife hand at the base of his arm with your left forearm, with your hand open. Raise his right elbow up as you turn your head to your left to get out of the headlock.