code: func (set IdentifierSet) Union(other IdentifierSet) IdentifierSet {
  	unionedSet := NewIdentifierSet()
  
  	for elem := range set {
  		unionedSet.Add(elem)
  	}
  	for elem := range other {
  		unionedSet.Add(elem)
  	}
  	return unionedSet
  }
docs: // Union returns a new set with all items in both sets.