docs: function checksum (bits, size) {
    if (bits == null || !isValid(bits)) throw new Error('bitString#checksum: Need valid bits');
    if (size == null || size % 2 !== 0) throw new Error('bitString#checksum: size needs to be even');
  
    var sumA = 0,
        sumB = 0,
        i;
  
    for (i = 0; i < bits.length; i++) {
      sumA = (sumA + parseInt(bits[i], 2)) % size;
      sumB = (sumB + sumA) % size;
    }
  
    return pad(sumB.toString(2), size / 2) + pad(sumA.toString(2), size / 2);
  }