code: func (a pgids) merge(b pgids) pgids {
  	// Return the opposite slice if one is nil.
  	if len(a) == 0 {
  		return b
  	}
  	if len(b) == 0 {
  		return a
  	}
  	merged := make(pgids, len(a)+len(b))
  	mergepgids(merged, a, b)
  	return merged
  }
docs: // merge returns the sorted union of a and b.