DRAFT: interactive tx construction protocol

In a discussion about the Lightning Network, it was suggested that multi-channel open would not be possible with PoDLE due to the signature committing to a specific node_id. However, it was later clarified that an initiator can use the same utxo as proof for multiple peers, but the signatures passed to each peer must commit to that specific peer's node_id. The revised PoDLE signature commitment requires every initiator to include at least one of their own inputs in the transaction and attempting to initiate an additional open using someone else's utxo won't work. Ideally, a node would remove the PoDLE TLV data from any 'forwarded' tx_add_inputs that isn't the input they're proving for, to prevent leaking information about which inputs belong to other parties. The post includes a conversation between ZmnSCPxj, Rusty, niftynei, and others discussing the protocol for multiparty constructions and defining collision between serial ids.