diff --git a/fold.go b/fold.go
index 717033f..d481be9 100644
--- a/fold.go
+++ b/fold.go
@@ -1,6 +1,6 @@
 package lambda
 
-func Foldl[T any](xs []T, fx func(T, T) T) T {
+func Foldr[T any](xs []T, fx func(T, T) T) T {
 	if len(xs) < 1 {
 		return *new(T)
 	}
@@ -13,7 +13,7 @@ func Foldl[T any](xs []T, fx func(T, T) T) T {
 	return xs[0]
 }
 
-func Foldr[T any](xs []T, fx func(T, T) T) T {
+func Foldl[T any](xs []T, fx func(T, T) T) T {
 	if len(xs) < 1 {
 		return *new(T)
 	}
diff --git a/go.mod b/go.mod
index 2dba43e..300c325 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,5 @@
 module git.tuxpa.in/a/lambda
 
 go 1.18
+
+require golang.org/x/exp v0.0.0-20220325121720-054d8573a5d8 // indirect
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..fe45005
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,2 @@
+golang.org/x/exp v0.0.0-20220325121720-054d8573a5d8 h1:Xt4/LzbTwfocTk9ZLEu4onjeFucl88iW+v4j4PWbQuE=
+golang.org/x/exp v0.0.0-20220325121720-054d8573a5d8/go.mod h1:lgLbSvA5ygNOMpwM/9anMpWVlVJ7Z+cHWq/eFuinpGE=
diff --git a/map_test.go b/map_test.go
new file mode 100644
index 0000000..37ecb53
--- /dev/null
+++ b/map_test.go
@@ -0,0 +1,17 @@
+package lambda_test
+
+import (
+	"reflect"
+	"testing"
+
+	"git.tuxpa.in/a/lambda"
+)
+
+func TestMapAdd(t *testing.T) {
+	arr := []int{1, 2, 3, 4, 6, 5}
+	ans := lambda.Map(arr, func(x int) int { return x + 4 })
+	exp := []int{5, 6, 7, 8, 10, 9}
+	if !reflect.DeepEqual(ans, exp) {
+		t.Fatalf("expect %v to be %v", ans, exp)
+	}
+}
diff --git a/math.go b/math.go
new file mode 100644
index 0000000..3eda2cb
--- /dev/null
+++ b/math.go
@@ -0,0 +1,23 @@
+package lambda
+
+import "golang.org/x/exp/constraints"
+
+type Numeric interface {
+	constraints.Complex | constraints.Integer | constraints.Float
+}
+
+func Add[T Numeric](a, b T) T {
+	return a + b
+}
+
+func Sub[T Numeric](a, b T) T {
+	return a - b
+}
+
+func Mul[T Numeric](a, b T) T {
+	return a * b
+}
+
+func Div[T Numeric](a, b T) T {
+	return a / b
+}
diff --git a/parallel.go b/parallel.go
deleted file mode 100644
index 39af049..0000000
--- a/parallel.go
+++ /dev/null
@@ -1,5 +0,0 @@
-package lambda
-
-
-
-
diff --git a/parallel/map.go b/parallel/map.go
new file mode 100644
index 0000000..f174048
--- /dev/null
+++ b/parallel/map.go
@@ -0,0 +1,36 @@
+package parallel
+
+import (
+	"sync"
+
+	"git.tuxpa.in/a/lambda"
+)
+
+func Map[T any](xs []T, fx func(T) T) []T {
+	spl := lambda.Split(xs, routineCount)
+	wg := new(sync.WaitGroup)
+	wg.Add(len(spl))
+	for i, v := range spl {
+		go func(ix int, vx []T) {
+			spl[ix] = lambda.Map(vx, fx)
+			wg.Done()
+		}(i, v)
+	}
+	wg.Wait()
+	return lambda.Flatten(spl)
+}
+
+func MapV[T, V any](xs []T, fx func(T) T) []V {
+	spl := lambda.Split(xs, routineCount)
+	wg := new(sync.WaitGroup)
+	wg.Add(len(spl))
+	tmp := make([][]V, len(spl))
+	for i, v := range spl {
+		go func(ix int, vx []T) {
+			spl[ix] = lambda.Map(vx, fx)
+			wg.Done()
+		}(i, v)
+	}
+	wg.Wait()
+	return lambda.Flatten(tmp)
+}
diff --git a/parallel/parallel.go b/parallel/parallel.go
index bec772f..e7f5385 100644
--- a/parallel/parallel.go
+++ b/parallel/parallel.go
@@ -11,36 +11,6 @@ var routineCount int = 1
 func SetRoutineCount(n int) {
 	routineCount = n
 }
-
-func Map[T any](xs []T, fx func(T) T) []T {
-	spl := lambda.Split(xs, routineCount)
-	wg := new(sync.WaitGroup)
-	wg.Add(len(spl))
-	for i, v := range spl {
-		go func(ix int, vx []T) {
-			spl[ix] = lambda.Map(vx, fx)
-			wg.Done()
-		}(i, v)
-	}
-	wg.Wait()
-	return lambda.Flatten(spl)
-}
-
-func MapV[T, V any](xs []T, fx func(T) T) []V {
-	spl := lambda.Split(xs, routineCount)
-	wg := new(sync.WaitGroup)
-	wg.Add(len(spl))
-	tmp := make([][]V, len(spl))
-	for i, v := range spl {
-		go func(ix int, vx []T) {
-			spl[ix] = lambda.Map(vx, fx)
-			wg.Done()
-		}(i, v)
-	}
-	wg.Wait()
-	return lambda.Flatten(tmp)
-}
-
 func Foldl[T any](xs []T, fx func(T, T) T) T {
 	spl := lambda.Split(xs, routineCount)
 	wg := new(sync.WaitGroup)
diff --git a/transform.go b/transform.go
index 9a5ad10..527c926 100644
--- a/transform.go
+++ b/transform.go
@@ -11,7 +11,7 @@ func Entries[K comparable, V any](m map[K]V) ([]K, []V) {
 }
 
 func Flatten[T any](xss [][]T) []T {
-	return Foldl(xss, Merge[T])
+	return Foldr(xss, Merge[T])
 }
 
 func Merge[T any](a, b []T) []T {