What is a Proof of Chain Membership (PoChM) in Kaspa?

A Proof of Chain Membership (PoChM) is a cryptographic proof that a specific block appeared in Kaspa's selected chain. In Kaspa's blockDAG, blocks can exist in parallel paths, so proving a block made it into the canonical ordering — the "selected chain" — is a distinct and non-trivial step on top of simply showing a transaction exists inside a block. PoChM is exactly that second step: the proof that the block itself is on the selected chain rather than a parallel branch. The name is pronounced like the Latin word "pacem." Understanding PoChM matters because it is the mechanism that allows anyone to independently verify a Kaspa transaction was genuinely confirmed, without trusting a third party.