Constant MASK_COUNT 

pub const MASK_COUNT: usize = WINTERNITZ_W; // 16usize

Number of randomization masks that can affect a HashSigsRS result.

Upstream allocates 67 masks, but the function key uses index zero and every chain transition uses only indices 1 through 15. Therefore indices 16 through 66 are unreachable and these 16 masks are exactly sufficient.