X -> X.
if it's "m-of-n" then it's "n", not "N"
HDPN, no?
sharing master public key is pointless

They would share the purpose key, not the master key. That is because the
purpose key cannot be derived from the master key. (It is hardened, and thus
requires the master private key to derive, which would never be shared).

Also I think we ought to consider changing the scheme to this:

m / purpose' / wallet_id' / cosigner_id / change / address_index

Suppose someone wanted to use the same master private key on many different
multisig wallets. Without a wallet_id, they would only be able to use one such
wallet. But with a wallet_id, they can use a new wallet_id for every new
wallet. Thus they can participate infinitely many multisig wallets (or nearly
so) with the same master key.
Merge pull request #1 from ryanxcharles/bip32-p2sh-multisig

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