diff --git a/arrange.go b/arrange.go
index 3a43057..edfdc17 100644
--- a/arrange.go
+++ b/arrange.go
@@ -42,6 +42,7 @@ func mtime(path string) (time.Time, error) {
 	return s.ModTime(), nil
 }
 
+// PrepOutput creates all possible content-address prefix directories.
 func PrepOutput(root string) error {
 	for i := 0; i <= 0xff; i++ {
 		dirname := filepath.Join(root, "content", fmt.Sprintf("%02x", i))
@@ -55,6 +56,7 @@ func PrepOutput(root string) error {
 	return nil
 }
 
+// Source returns sends all files that match known extensions.
 func Source(root string) <-chan string {
 	out := make(chan string)
 	go func() {
@@ -82,6 +84,10 @@ func Source(root string) <-chan string {
 	return out
 }
 
+// Parse runs the file parser for each file on input chan, and sends results
+// down output chan.
+//
+// Exists so that it can be called many times concurrently.
 func Parse(in <-chan string) <-chan Media {
 	out := make(chan Media)
 	go func() {
@@ -105,6 +111,8 @@ func Parse(in <-chan string) <-chan Media {
 	return out
 }
 
+// Move calls Move on each Media on input chan. It is the first step in the
+// pipeline after fan-in.
 func Move(in <-chan Media, root string) <-chan error {
 	out := make(chan error)
 	go func() {
@@ -198,6 +206,7 @@ func _parse(path string) (Media, error) {
 	return r, nil
 }
 
+// Merge implements fan-in.
 func Merge(cs []<-chan Media) <-chan Media {
 	out := make(chan Media)
 	var wg sync.WaitGroup
diff --git a/arrange_test.go b/arrange_test.go
index a24ddda..03fa1d3 100644
--- a/arrange_test.go
+++ b/arrange_test.go
@@ -194,8 +194,8 @@ func TestMoveCollision(t *testing.T) {
 }
 
 func TestSundry(t *testing.T) {
-	fmt.Sprintf("%v", NotMedia{"hi"})
-	fmt.Sprintf("%v", Dup{"hi"})
+	_ = fmt.Sprintf("%v", NotMedia{"hi"})
+	_ = fmt.Sprintf("%v", Dup{"hi"})
 }
 
 func TestFlow(t *testing.T) {
@@ -272,7 +272,7 @@ func TestFlow(t *testing.T) {
 
 	for err := range Move(Merge(streams), tmp) {
 		if err != nil {
-			t.Errorf("unexpected error: %v")
+			t.Errorf("unexpected error: %v", err)
 		}
 	}
 
diff --git a/errors.go b/errors.go
index 3465d2c..8d5996d 100644
--- a/errors.go
+++ b/errors.go
@@ -2,6 +2,7 @@ package arrange
 
 import "fmt"
 
+// NotMedia is for unkown filetypes.
 type NotMedia struct {
 	Path string
 }
@@ -10,6 +11,7 @@ func (nm NotMedia) Error() string {
 	return fmt.Sprintf("not media: %q", nm.Path)
 }
 
+// Dup indicates a file with duplicate content.
 type Dup struct {
 	Path string
 }
diff --git a/media.go b/media.go
index 25f0fbe..6a5743b 100644
--- a/media.go
+++ b/media.go
@@ -8,6 +8,7 @@ import (
 	"time"
 )
 
+// Media is the high-level filetype that can be arranged.
 type Media struct {
 	Path      string
 	Hash      string
@@ -15,6 +16,8 @@ type Media struct {
 	Time      time.Time
 }
 
+// Move is called to push Media into its final destination, by content address
+// and by date.
 func (m Media) Move(root string) error {
 	f, err := os.Open(m.Path)
 	if err != nil {