+ /// Validate the counterparty's signatures on the holder commitment transaction and HTLCs.
+ ///
+ /// This is required in order for the signer to make sure that releasing a commitment
+ /// secret won't leave us without a broadcastable holder transaction.
+ /// Policy checks should be implemented in this function, including checking the amount
+ /// sent to us and checking the HTLCs.
+ fn validate_holder_commitment(&self, holder_tx: &HolderCommitmentTransaction) -> Result<(), ()>;