diff --git a/CHANGELOG.md b/CHANGELOG.md
index 56f0c39..8e9fce3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [v0.12.2] - UNRELEASED
+
+### Added
+
+- pkg/sets: add Intersect() and Add() methods.
+
 ## [v0.12.1] - 2024-04-03
 
 ### Minor breaking change
diff --git a/go.mod b/go.mod
index 59371ca..b61e843 100644
--- a/go.mod
+++ b/go.mod
@@ -5,7 +5,7 @@ go 1.22
 require (
 	dario.cat/mergo v1.0.0
 	github.com/alexflint/go-arg v1.4.3
-	github.com/go-quicktest/qt v1.101.0
+	github.com/go-quicktest/qt v1.101.1-0.20240301121107-c6c8733fa1e6
 	github.com/google/go-cmp v0.6.0
 	github.com/sasbury/mini v0.0.0-20181226232755-dc74af49394b
 	gotest.tools/v3 v3.5.1
diff --git a/go.sum b/go.sum
index 6926d49..4516abd 100644
--- a/go.sum
+++ b/go.sum
@@ -9,8 +9,8 @@ github.com/creack/pty v1.1.9/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ3
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
-github.com/go-quicktest/qt v1.101.0 h1:O1K29Txy5P2OK0dGo59b7b0LR6wKfIhttaAhHUyn7eI=
-github.com/go-quicktest/qt v1.101.0/go.mod h1:14Bz/f7NwaXPtdYEgzsx46kqSxVwTbzVZsDC26tQJow=
+github.com/go-quicktest/qt v1.101.1-0.20240301121107-c6c8733fa1e6 h1:teYtXy9B7y5lHTp8V9KPxpYRAVA7dozigQcMiBust1s=
+github.com/go-quicktest/qt v1.101.1-0.20240301121107-c6c8733fa1e6/go.mod h1:p4lGIVX+8Wa6ZPNDvqcxq36XpUDLh42FLetFU7odllI=
 github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
 github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
diff --git a/sets/sets.go b/sets/sets.go
index 82863c1..3c892f8 100644
--- a/sets/sets.go
+++ b/sets/sets.go
@@ -60,6 +60,15 @@ func (s *Set[T]) Contains(item T) bool {
 	return found
 }
 
+// Add inserts item into s. Returns true if the item was present.
+func (s *Set[T]) Add(item T) bool {
+	if s.Contains(item) {
+		return true
+	}
+	s.items[item] = struct{}{}
+	return false
+}
+
 // Remove deletes item from s. Returns true if the item was present.
 func (s *Set[T]) Remove(item T) bool {
 	if !s.Contains(item) {
diff --git a/sets/sets_test.go b/sets/sets_test.go
index 3a68e63..32c3fa5 100644
--- a/sets/sets_test.go
+++ b/sets/sets_test.go
@@ -4,7 +4,7 @@ import (
 	"fmt"
 	"testing"
 
-	"gotest.tools/v3/assert"
+	"github.com/go-quicktest/qt"
 
 	"github.com/Pix4D/cogito/sets"
 )
@@ -22,9 +22,9 @@ func TestFromInt(t *testing.T) {
 		s := sets.From(tc.items...)
 		sorted := s.OrderedList()
 
-		assert.Equal(t, s.Size(), tc.wantSize)
-		assert.DeepEqual(t, sorted, tc.wantList)
-		assert.Equal(t, fmt.Sprint(s), tc.wantString)
+		qt.Assert(t, qt.Equals(s.Size(), tc.wantSize))
+		qt.Assert(t, qt.DeepEquals(sorted, tc.wantList))
+		qt.Assert(t, qt.Equals(fmt.Sprint(s), tc.wantString))
 	}
 
 	testCases := []testCase{
@@ -69,9 +69,9 @@ func TestFromString(t *testing.T) {
 		s := sets.From(tc.items...)
 		sorted := s.OrderedList()
 
-		assert.Equal(t, s.Size(), tc.wantSize)
-		assert.DeepEqual(t, sorted, tc.wantList)
-		assert.Equal(t, fmt.Sprint(s), tc.wantString)
+		qt.Assert(t, qt.Equals(s.Size(), tc.wantSize))
+		qt.Assert(t, qt.DeepEquals(sorted, tc.wantList))
+		qt.Assert(t, qt.Equals(fmt.Sprint(s), tc.wantString))
 	}
 
 	testCases := []testCase{
@@ -101,7 +101,7 @@ func TestDifference(t *testing.T) {
 		result := tc.s.Difference(tc.x)
 		sorted := result.OrderedList()
 
-		assert.DeepEqual(t, sorted, tc.wantList)
+		qt.Assert(t, qt.DeepEquals(sorted, tc.wantList))
 	}
 
 	testCases := []testCase{
@@ -154,7 +154,7 @@ func TestIntersection(t *testing.T) {
 		result := tc.s.Intersection(tc.x)
 		sorted := result.OrderedList()
 
-		assert.DeepEqual(t, sorted, tc.wantList)
+		qt.Assert(t, qt.DeepEquals(sorted, tc.wantList))
 	}
 
 	testCases := []testCase{
@@ -214,8 +214,8 @@ func TestRemoveFound(t *testing.T) {
 
 		found := s.Remove(tc.remove)
 
-		assert.DeepEqual(t, s.OrderedList(), tc.wantList)
-		assert.Assert(t, found)
+		qt.Assert(t, qt.DeepEquals(s.OrderedList(), tc.wantList))
+		qt.Assert(t, qt.IsTrue(found))
 	}
 
 	testCases := []testCase{
@@ -250,8 +250,8 @@ func TestRemoveNotFound(t *testing.T) {
 
 		found := s.Remove(tc.remove)
 
-		assert.DeepEqual(t, s.OrderedList(), tc.items)
-		assert.Assert(t, !found)
+		qt.Assert(t, qt.DeepEquals(s.OrderedList(), tc.items))
+		qt.Assert(t, qt.IsFalse(found))
 	}
 
 	testCases := []testCase{
@@ -271,3 +271,41 @@ func TestRemoveNotFound(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) { test(t, tc) })
 	}
 }
+
+func TestAdd(t *testing.T) {
+	type testCase struct {
+		name     string
+		items    []int
+		wantList []int
+	}
+
+	test := func(t *testing.T, tc testCase) {
+		s := sets.New[int](5)
+		for _, item := range tc.items {
+			s.Add(item)
+		}
+		qt.Assert(t, qt.DeepEquals(s.OrderedList(), tc.wantList))
+	}
+
+	testCases := []testCase{
+		{
+			name:     "one item",
+			items:    []int{3},
+			wantList: []int{3},
+		},
+		{
+			name:     "multiple items",
+			items:    []int{3, 0, 42},
+			wantList: []int{0, 3, 42},
+		},
+		{
+			name:     "duplicates",
+			items:    []int{10, 5, 5, 10, 1},
+			wantList: []int{1, 5, 10},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) { test(t, tc) })
+	}
+}