Cool idea! It makes sense to reserve all negative numbers for private kinds since there could be any number of private kinds. 

However, I wonder if there are any other things that also need their own space. Maybe a binary prefix (+ or -) limits the number of orthogonal kinds a bit much.

In Bitcoin we use `bc1` to indicate that an address is a bech32 main net address and `tb1` to indicate that it's a bech32 testnet addresses. 

I wonder if we can introduce similar prefixes for nostr notes or signatures...