code: func (b *Bitset) ensureCapacity(numBits int) {
  	numBits += b.numBits
  
  	newNumBytes := numBits / 8
  	if numBits%8 != 0 {
  		newNumBytes++
  	}
  
  	if len(b.bits) >= newNumBytes {
  		return
  	}
  
  	b.bits = append(b.bits, make([]byte, newNumBytes+2*len(b.bits))...)
  }
docs: // ensureCapacity ensures the Bitset can store an additional |numBits|.
  //
  // The underlying array is expanded if necessary. To prevent frequent
  // reallocation, expanding the underlying array at least doubles its capacity.