+ /// Signs a transaction created by build_htlc_transaction. If the transaction is an
+ /// HTLC-Success transaction, preimage must be set!
+ /// TODO: should be merged with sign_local_commitment as a slice of HTLC transactions to sign
+ fn sign_htlc_transaction<T: secp256k1::Signing>(&self, local_commitment_tx: &mut LocalCommitmentTransaction, htlc_index: u32, preimage: Option<PaymentPreimage>, local_csv: u16, secp_ctx: &Secp256k1<T>);