diff --git a/ledger/file.go b/ledger/file.go
index c9d060a..cdc3d99 100644
--- a/ledger/file.go
+++ b/ledger/file.go
@@ -10,7 +10,7 @@ func NewFiles(p string, q ...string) (Files, error) {
 	return f, err
 }
 
-func (files Files) Add(delta Delta) error {
+func (files Files) Add(payee string, delta Delta) error {
 	//   TODO
 	return io.EOF
 }
diff --git a/ledger/file_test.go b/ledger/file_test.go
index 7348b7d..f32f797 100644
--- a/ledger/file_test.go
+++ b/ledger/file_test.go
@@ -1,10 +1,94 @@
 package ledger
 
 import (
+	"os"
+	"path"
 	"path/filepath"
 	"testing"
 )
 
+func TestFileAdd(t *testing.T) {
+	payee := "name:3"
+	delta := Delta{
+		Date:        "2999-88-77",
+		Description: "66",
+		Name:        "name:1",
+		Value:       2.00,
+		Currency:    USD,
+	}
+	cases := map[string]struct {
+		given []byte
+		want  string
+	}{
+		"no file": {
+			given: nil,
+			want: `2999-88-77 66
+  name:1 $2.00
+  name:3`,
+		},
+		"empty file": {
+			given: []byte{},
+			want: `2999-88-77 66
+  name:1 $2.00
+  name:3`,
+		},
+		"happy without trailing whitespace": {
+			given: []byte(`
+2000-01-02 desc
+  name:1 $1.00
+  name:2 $-1.00`),
+			want: `
+2000-01-02 desc
+  name:1 $1.00
+  name:2 $-1.00
+2999-88-77 66
+  name:1 $2.00
+  name:3`,
+		},
+		"happy with trailing newline": {
+			given: []byte(`
+2000-01-02 desc
+  name:1 $1.00
+  name:2 $-1.00
+         `),
+			want: `
+2000-01-02 desc
+  name:1 $1.00
+  name:2 $-1.00
+2999-88-77 66
+  name:1 $2.00
+  name:3`,
+		},
+	}
+
+	for name, d := range cases {
+		c := d
+		t.Run(name, func(t *testing.T) {
+			p := path.Join(t.TempDir(), "input")
+			if c.given != nil {
+				if err := os.WriteFile(p, []byte(c.given), os.ModePerm); err != nil {
+					t.Fatal(err)
+				}
+			}
+
+			f, err := NewFiles(p)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			if err := f.Add(payee, delta); err != nil {
+				t.Fatal(err)
+			}
+
+			if got, err := os.ReadFile(p); err != nil {
+				t.Fatal(err)
+			} else if string(got) != c.want {
+				t.Errorf("wanted\n\t%s, got\n\t%s", c.want, got)
+			}
+		})
+	}
+}
+
 func TestFileTestdata(t *testing.T) {
 	t.Run("macro.d", func(t *testing.T) {
 		paths, err := filepath.Glob("./testdata/macro.d/*")