Hey all,

I wanted to propose a new key in the global context for BIP174,
Partially-Signed Bitcoin Transactions.

= Rationale

Each signer should make sure that the inputs being referenced in the PSBT
exist (with the exception of a Proof-of-Reserves input). In order to do
this, it's critical to know which network the coins are on (mainnet or
testnet). This could potentially be extended to other networks should they
want to use something like PSBT, much in the same way that HD keys from
BIP0044 reserved 0' and 1' as coins for mainnet Bitcoin and testnet Bitcoin
respectively.

= Proposal

Add the key 0x03 for network in the global key-value store. Value is a
variable integer with 0x00 indicating Bitcoin mainnet and 0x01 indicating
Bitcoin testnet. Other coins that want to use the PSBT should use the coin
network number from SLIP-0044 with the high bit removed.

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Best,

Jimmy