code: func (b *Bitset) Append(other *Bitset) {
  	b.ensureCapacity(other.numBits)
  
  	for i := 0; i < other.numBits; i++ {
  		if other.At(i) {
  			b.bits[b.numBits/8] |= 0x80 >> uint(b.numBits%8)
  		}
  		b.numBits++
  	}
  }
docs: // Append bits copied from |other|.
  //
  // The new length is b.Len() + other.Len().