Free Transactions Using a Precompiled Contract about rskips HOT 2 OPEN

The specification suggests that the payload format is:
payload = (nonce,signature, (list_of_intentions) )
where
list_of_intentions = list of (receiver address, value, data)

What does the signature signs ? There are several possibilities. I think you mean the payload but replacing the signature by something else. It is the empty field?

from rskips.

The use of the account nonce in a non-free transaction brings several problems regarding mempool.
One can think of an attack where the attacker fills the mempool with a huge transaction (and the network propagates it) but the attacker cancels it with a single call to the precompiled contract, where the call was hidden in another previous transaction in the block, under multiple recursive calls ?
To be more clear, both the call to the precompile and the free-transaction call share the same nonce.
This attack can be repeated with hundreds of accounts simultaneously, as a DoS attack to miners and the network.
That's why Patrick McCorry's proposal uses a different nonce queue.

from rskips.