Compact proof section

[dimartiro]

Hi, I'm part of the chainsafe team that are implementing the Gossamer client.

In the past weeks I've been working on implementing the state trie V1 and I found the "compact proof" implementation in substrate but it is not in the specification.

My question:

Is this used only in substrate or is part of the spec? Is there more information about this type of proofs? Can you add more details in the spec?

Thanks!