diff --git a/src/build/expression/Expression.go b/src/build/expression/Expression.go
index 84b9249..c2be790 100644
--- a/src/build/expression/Expression.go
+++ b/src/build/expression/Expression.go
@@ -26,15 +26,6 @@ func NewLeaf(t token.Token) *Expression {
 	return expr
 }
 
-// NewBinary creates a new binary operator expression.
-func NewBinary(left *Expression, operator token.Token, right *Expression) *Expression {
-	expr := New()
-	expr.Token = operator
-	expr.AddChild(left)
-	expr.AddChild(right)
-	return expr
-}
-
 // AddChild adds a child to the expression.
 func (expr *Expression) AddChild(child *Expression) {
 	expr.Children = append(expr.Children, child)
diff --git a/src/build/expression/Expression_test.go b/src/build/expression/Expression_test.go
index 62ce2d9..c0fe259 100644
--- a/src/build/expression/Expression_test.go
+++ b/src/build/expression/Expression_test.go
@@ -1,6 +1,7 @@
 package expression_test
 
 import (
+	"fmt"
 	"testing"
 
 	"git.akyoto.dev/cli/q/src/build/expression"
@@ -95,12 +96,47 @@ func TestExpressionParse(t *testing.T) {
 	}
 }
 
-func BenchmarkExpression(b *testing.B) {
-	src := []byte("(1+2-3*4)+(5*6-7+8)\n")
+func TestEachLeaf(t *testing.T) {
+	src := []byte("(1+2-3*4)+(5*6-7+8)")
 	tokens := token.Tokenize(src)
+	expr := expression.Parse(tokens)
+	leaves := []string{}
 
-	for i := 0; i < b.N; i++ {
-		expr := expression.Parse(tokens)
-		expr.Close()
-	}
+	expr.EachLeaf(func(leaf *expression.Expression) error {
+		leaves = append(leaves, leaf.Token.Text())
+		return nil
+	})
+
+	assert.DeepEqual(t, leaves, []string{"1", "2", "3", "4", "5", "6", "7", "8"})
+
+	err := expr.EachLeaf(func(leaf *expression.Expression) error {
+		return fmt.Errorf("error")
+	})
+
+	assert.NotNil(t, err)
+	assert.Equal(t, err.Error(), "error")
+}
+
+func TestRemoveChild(t *testing.T) {
+	src := []byte("(1+2-3*4)+(5*6-7+8)")
+	tokens := token.Tokenize(src)
+	expr := expression.Parse(tokens)
+	left := expr.Children[0]
+	right := expr.Children[1]
+	expr.RemoveChild(left)
+	assert.Equal(t, expr.Children[0], right)
+}
+
+func TestNilExpression(t *testing.T) {
+	src := []byte("")
+	tokens := token.Tokenize(src)
+	expr := expression.Parse(tokens)
+	assert.Nil(t, expr)
+}
+
+func TestNilGroup(t *testing.T) {
+	src := []byte("()")
+	tokens := token.Tokenize(src)
+	expr := expression.Parse(tokens)
+	assert.Nil(t, expr)
 }
diff --git a/src/build/expression/bench_test.go b/src/build/expression/bench_test.go
new file mode 100644
index 0000000..8287416
--- /dev/null
+++ b/src/build/expression/bench_test.go
@@ -0,0 +1,18 @@
+package expression_test
+
+import (
+	"testing"
+
+	"git.akyoto.dev/cli/q/src/build/expression"
+	"git.akyoto.dev/cli/q/src/build/token"
+)
+
+func BenchmarkExpression(b *testing.B) {
+	src := []byte("(1+2-3*4)+(5*6-7+8)")
+	tokens := token.Tokenize(src)
+
+	for i := 0; i < b.N; i++ {
+		expr := expression.Parse(tokens)
+		expr.Close()
+	}
+}