code: func (b *Bitmap) Union(others ...*Bitmap) *Bitmap {
  	if len(others) == 1 {
  		output := NewBitmap()
  		b.unionIntoTargetSingle(output, others[0])
  		return output
  	}
  	output := b.Clone()
  	output.UnionInPlace(others...)
  	return output
  }
docs: // Union returns the bitwise union of b and others as a new bitmap.