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)).