Lemma HMAC_DRBG_generate_helper_Z_incremental_fst:
forall HMAC key v z v',
0 <= z ->
v' = fst (HMAC_DRBG_generate_helper_Z HMAC key v z) ->
HMAC v' key = fst (HMAC_DRBG_generate_helper_Z HMAC key v (z + 32)).