docs: function hex2a(hex) {
    return Array.from(hex)
      .map(s => {
        if (s >= "0" && s <= "9") {
          return String.fromCharCode(s.charCodeAt(0) + N_TO_A);
        } else if (s >= "a" && s <= "f") {
          return String.fromCharCode(s.charCodeAt(0) + A_TO_K);
        }
        throw new Error(`invalid char: ${s}`);
      })
      .join("");
  }