What is the PoChM Merkle root (PMR) in a Kaspa block header?

The PoChM Merkle root (PMR) is a proposed new field in Kaspa block headers that encodes a compact cryptographic summary of a block's chain ancestry. In Kaspa's blockDAG, tracing a block's full ancestry can require fetching many headers across the network. The PMR shortens this by storing the root of a Merkle tree built from select ancestor headers at exponentially spaced intervals, so a node can verify ancestry with a small proof instead of the full chain. For a beginner, the PMR is the building block that lets Kaspa nodes confirm a block belongs to the selected chain quickly and without downloading every ancestor block.