diff --git a/cmd/transpile/main.go b/cmd/transpile/main.go
index e8f8331..f4a397c 100644
--- a/cmd/transpile/main.go
+++ b/cmd/transpile/main.go
@@ -39,7 +39,8 @@ func usage(w io.Writer) {
 	fmt.Fprintln(w, "")
 	fmt.Fprintln(w, " mothball: Compile a mothball")
 	fmt.Fprintln(w, " inventory: Show category inventory")
-	fmt.Fprintln(w, " open: Open a file for a puzzle")
+	fmt.Fprintln(w, " puzzle: Print puzzle JSON")
+	fmt.Fprintln(w, " file: Open a file for a puzzle")
 	fmt.Fprintln(w, " answer: Check correctness of an answer")
 }
 
@@ -62,7 +63,9 @@ func (t *T) ParseArgs() (Command, error) {
 		flags.StringVar(&t.filename, "out", "", "Path to create mothball (empty for stdout)")
 	case "inventory":
 		cmd = t.PrintInventory
-	case "open":
+	case "puzzle":
+		cmd = t.DumpPuzzle
+	case "file":
 		cmd = t.DumpFile
 		flags.StringVar(&t.filename, "file", "puzzle.json", "Filename to open")
 	case "answer":
@@ -112,31 +115,33 @@ func (t *T) PrintInventory() error {
 	return nil
 }
 
+// DumpPuzzle writes a puzzle's JSON to the writer.
+func (t *T) DumpPuzzle() error {
+	puzzle := transpile.NewFsPuzzle(t.fs)
+
+	p, err := puzzle.Puzzle()
+	if err != nil {
+		return err
+	}
+	jp, err := json.Marshal(p)
+	if err != nil {
+		return err
+	}
+	t.Stdout.Write(jp)
+	return nil
+}
+
 // DumpFile writes a file to the writer.
-// BUG(neale): The "open" and "answer" actions don't work on categories with an "mkcategory" executable.
 func (t *T) DumpFile() error {
 	puzzle := transpile.NewFsPuzzle(t.fs)
 
-	switch t.filename {
-	case "puzzle.json", "":
-		p, err := puzzle.Puzzle()
-		if err != nil {
-			return err
-		}
-		jp, err := json.Marshal(p)
-		if err != nil {
-			return err
-		}
-		t.Stdout.Write(jp)
-	default:
-		f, err := puzzle.Open(t.filename)
-		if err != nil {
-			return err
-		}
-		defer f.Close()
-		if _, err := io.Copy(t.Stdout, f); err != nil {
-			return err
-		}
+	f, err := puzzle.Open(t.filename)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	if _, err := io.Copy(t.Stdout, f); err != nil {
+		return err
 	}
 
 	return nil
@@ -174,12 +179,10 @@ func (t *T) DumpMothball() error {
 // CheckAnswer prints whether an answer is correct.
 func (t *T) CheckAnswer() error {
 	c := transpile.NewFsPuzzle(t.fs)
-	if c.Answer(t.answer) {
-		fmt.Fprintln(t.Stdout, "correct")
-	} else {
-		fmt.Fprintln(t.Stdout, "wrong")
-	}
-	return nil
+	log.Print(c.Puzzle())
+	log.Print(t.answer)
+	_, err := fmt.Fprintf(t.Stdout, `{"Correct":%v}`, c.Answer(t.answer))
+	return err
 }
 
 func main() {
diff --git a/cmd/transpile/main_test.go b/cmd/transpile/main_test.go
index 52e1a1e..5566018 100644
--- a/cmd/transpile/main_test.go
+++ b/cmd/transpile/main_test.go
@@ -50,7 +50,7 @@ func (tp T) Run(args ...string) error {
 	return command()
 }
 
-func TestEverything(t *testing.T) {
+func TestTranspilerEverything(t *testing.T) {
 	stdout := new(bytes.Buffer)
 	stderr := new(bytes.Buffer)
 	tp := T{
@@ -67,7 +67,7 @@ func TestEverything(t *testing.T) {
 	}
 
 	stdout.Reset()
-	if err := tp.Run("open", "-dir=cat0/1"); err != nil {
+	if err := tp.Run("puzzle", "-dir=cat0/1"); err != nil {
 		t.Error(err)
 	}
 	p := transpile.Puzzle{}
@@ -79,12 +79,21 @@ func TestEverything(t *testing.T) {
 	}
 
 	stdout.Reset()
-	if err := tp.Run("open", "-dir=cat0/1", "-file=moo.txt"); err != nil {
+	if err := tp.Run("file", "-dir=cat0/1", "-file=moo.txt"); err != nil {
 		t.Error(err)
 	}
 	if stdout.String() != "Moo." {
 		t.Error("Wrong file pulled", stdout.String())
 	}
+
+	stdout.Reset()
+	if err := tp.Run("answer", "-dir=cat0/1", "-answer=YAML answer"); err != nil {
+		t.Error(err)
+	}
+	if stdout.String() != `{"Correct":true}` {
+		t.Error("Answer validation failed", stdout.String())
+	}
+
 }
 
 func TestMothballs(t *testing.T) {
diff --git a/example-puzzles/mkcategory/mkcategory b/example-puzzles/mkcategory/mkcategory
new file mode 100755
index 0000000..fff262d
--- /dev/null
+++ b/example-puzzles/mkcategory/mkcategory
@@ -0,0 +1,33 @@
+#! /bin/sh
+
+fail () {
+    echo "ERROR: $*" 1>&2
+    exit 1
+}
+
+case $1 in
+    inventory)
+        printf '{"Puzzles":['
+        ls  -d */[0-9]* | while read p; do
+            puzzle=${p##*/}
+            printf "%s%d,10%d" "$comma" "$puzzle" "$puzzle"
+            comma=,
+        done
+        printf ']}'
+        ;;
+    puzzle)
+        points=$(($2 % 100))
+        transpile puzzle -dir */$points
+        ;;
+    file)
+        points=$(($2 % 100))
+        transpile file -dir */$points "$3"
+        ;;
+    answer)
+        points=$(($2 % 100))
+        transpile answer -dir */$points -answer="$3"
+        ;;
+    *)
+        fail "What is $1" 1>&2
+        ;;
+esac
diff --git a/example-puzzles/mkcategory/subcat1/1/puzzle.md b/example-puzzles/mkcategory/subcat1/1/puzzle.md
new file mode 100644
index 0000000..18d2e72
--- /dev/null
+++ b/example-puzzles/mkcategory/subcat1/1/puzzle.md
@@ -0,0 +1,10 @@
+---
+pre:
+    authors: ["neale"]
+answers: ["6"]
+---
+This category shows off how to use the transpiler from an
+`mkcategory` script to organize puzzles into sub-categories,
+and (pointlessly) duplicate every puzzle in the category.
+
+    1 2 3 4 5 _
diff --git a/example-puzzles/mkcategory/subcat1/2/puzzle.md b/example-puzzles/mkcategory/subcat1/2/puzzle.md
new file mode 100644
index 0000000..8c4756b
--- /dev/null
+++ b/example-puzzles/mkcategory/subcat1/2/puzzle.md
@@ -0,0 +1,6 @@
+---
+pre:
+    authors: ["neale"]
+answers: ["C", "c"]
+---
+    2 4 6 8 A _
diff --git a/example-puzzles/mkcategory/subcat2/3/puzzle.md b/example-puzzles/mkcategory/subcat2/3/puzzle.md
new file mode 100644
index 0000000..151347b
--- /dev/null
+++ b/example-puzzles/mkcategory/subcat2/3/puzzle.md
@@ -0,0 +1,6 @@
+---
+pre:
+    authors: ["neale"]
+answers: ["17"]
+---
+    1 2 3 5 7 11 13 _