to test this you'll need 4 lightningds running: 2 on bitcoin regtest, 2 on liquid.

A: regtest B: regtest C: liquid D: liquid

1. copy hsm_secret from C into B so they have the same nodeid.
2. install bridge in both B and C.
3. install solidifier in D.
4. set tcll-other-rpc on B to the lightning-rpc path of C.
5. set tcll-other-rpc on C to the lightning-rpc path of B.
6. set tcll-bridge-id on D to the nodeid of C.
7. open a channel between A and B and another channel between C and D.

now you can use bitcoin_invoice to generate invoices on D that can be paid by A

and use bitcoin_pay on D to pay invoices generated by A.