Function node_address_kernel 

pub fn node_address_kernel(ring: b5, level: b3, index: KeyIndex) -> b10

Node address of (level, index) in ring entry ring: ring · NODES_PER_ENTRY + NODE_BASE[level] + index.

Used by the builder (writes and child reads) and by the signer (sibling reads, through sibling_address_kernel). The multiply is a shift because NODES_PER_ENTRY = 2^(SUBTREE_LEVELS + 1).