Constant KEY_BITS 

pub const KEY_BITS: usize = _; // 6usize

Width of a key’s number within its slice’s batch share (j) and of a Merkle node’s index within its level: 6 bits in every batch-engine build (up to 64 keys per slice), more only when a slice makes larger batch shares. Kept at 6 rather than shrunk so the measured builds’ hardware does not change.