code: func (c *Container) setBitmap(bitmap []uint64) {
  	if roaringParanoia {
  		if c.typ != containerBitmap {
  			panic("attempt to write non-bitmap's bitmap")
  		}
  	}
  	h := (*reflect.SliceHeader)(unsafe.Pointer(&bitmap))
  	c.pointer, c.len, c.cap = (*uint16)(unsafe.Pointer(h.Data)), int32(h.Len), int32(h.Cap)
  	runtime.KeepAlive(&bitmap)
  }
docs: // setBitmap stores a set of uint64s as data.