Definition ack_in_flight_to_primary (l : list packet) : nat :=
length (filterMap
(fun p => if msg_eq_dec (pBody p) ack
then if name_eq_dec (pDst p) primary
then Some tt else None
else None)
l).

Lemma ack_in_flight_to_primary_backup :
forall p,
pDst p = backup ->
ack_in_flight_to_primary [p] = 0.