clamp(a, len) {
			if (a.length * 32 < len) {
				return a;
			}
			a = a.slice(0, Math.ceil(len / 32));
			const l = a.length;
			len = len & 31;
			if (l > 0 && len) {
				a[l - 1] = bitArray.partial(len, a[l - 1] & 0x80000000 >> (len - 1), 1);
			}
			return a;
		}
Truncate an array.
@param {bitArray} a The array.
@param {Number} len The length to truncate to, in bits.
@return {bitArray} A new array, truncated to len bits.