code: func (b *Bitmap) directOpN(op func(c *Container, v uint16) bool, a ...uint64) (changed int) {
  	hb := uint64(0xFFFFFFFFFFFFFFFF) // impossible sentinel value
  	var cont *Container
  	for _, v := range a {
  		if newhb := highbits(v); newhb != hb {
  			hb = newhb
  			cont = b.Containers.GetOrCreate(hb)
  		}
  		if op(cont, lowbits(v)) {
  			a[changed] = v
  			changed++
  		}
  	}
  	return changed
  }
docs: // directOpN contains the logic for DirectAddN and DirectRemoveN. Theoretically,
  // it could be used by anything that wanted to apply a boolean-returning
  // container level operation across a list of values and return the number of
  // trues while modifying the list of values in place to contain the
  // true-returning values in order.