code: func Copy(src PackedIntsReader, srcPos int, dest Mutable, destPos, length, mem int) {
  	assert(srcPos+length <= int(src.Size()))
  	assert(destPos+length <= int(dest.Size()))
  	capacity := int(uint(mem) >> 3)
  	if capacity == 0 {
  		panic("niy")
  	} else if length > 0 {
  		// use bulk operations
  		if length < capacity {
  			capacity = length
  		}
  		buf := make([]int64, capacity)
  		copyWith(src, srcPos, dest, destPos, length, buf)
  	}
  }
docs: /* Copy src[srcPos:srcPos+len] into dest[destPos:destPos+len] using at most mem bytes. */