add copying function

set.go

@@ -1,5 +1,7 @@
 package stringset
 
+import "maps"
+
 type StringSet struct {
 	set map[string]bool
 }
@@ -36,6 +38,14 @@ func (s *StringSet) All() []string {
 	return all
 }
 
+func (s *StringSet) Copy() *StringSet {
+	newSet := make(map[string]bool, 0)
+	maps.Copy(newSet, s.set)
+	return &StringSet{
+		set: newSet,
+	}
+}
+
 func (s *StringSet) Contains(str string) bool {
 	_, ok := s.set[str]
 	return ok

set_test.go

@@ -79,10 +79,20 @@ func TestNewStringSetWith(t *testing.T) {
 	}
 
 	s.Add("foo").Add("qux")
-	// s = noo, lol
+	// s = noo, lol, foo, qux
 	if !s.Equals(NewStringSetWith([]string{"noo", "lol", "foo", "qux"})) {
 		t.Errorf("unexpected result from Add() chaining")
 	}
+
+	s2 = s.Copy().Remove("noo")
+	// s = lol, foo, qux
+	if !s2.Equals(NewStringSetWith([]string{"lol", "foo", "qux"})) {
+		t.Errorf("unexpected result after cloning and removing")
+	}
+
+	if !s.Equals(NewStringSetWith([]string{"noo", "lol", "foo", "qux"})) {
+		t.Errorf("unexpected result original was modified via the clone")
+	}
 }
 
 func stringSlicesMatch(s1, s2 []string) bool {