code: func (b *Bitmap) DirectAddN(a ...uint64) (changed int) {
  	return b.directOpN((*Container).add, a...)
  }
docs: // DirectAddN sets multiple bits in the bitmap, returning how many changed. It
  // modifies the slice 'a' in place such that once it's complete a[:changed] will
  // be list of changed bits. It is more efficient than repeated calls to
  // DirectAdd for semi-dense sorted data because it reuses the container from the
  // previous value if the new value has the same highbits instead of looking it
  // up each time. TODO: if Containers implementations cached the last few
  // Container objects returned from calls like Get and GetOrCreate, this
  // optimization would be less useful.