diff --git a/cmd/transpile/basepath.go b/cmd/transpile/basepath.go
index 4dddc40..722be1c 100644
--- a/cmd/transpile/basepath.go
+++ b/cmd/transpile/basepath.go
@@ -9,24 +9,24 @@ import (
 	"github.com/spf13/afero"
 )
 
-// BasePathFs is an overloaded afero.BasePathFs that has a recursive RealPath().
-type BasePathFs struct {
+// RecursiveBasePathFs is an overloaded afero.BasePathFs that has a recursive RealPath().
+type RecursiveBasePathFs struct {
 	afero.Fs
 	source afero.Fs
 	path   string
 }
 
-// NewBasePathFs returns a new BasePathFs.
-func NewBasePathFs(source afero.Fs, path string) afero.Fs {
-	return &BasePathFs{
+// NewRecursiveBasePathFs returns a new RecursiveBasePathFs.
+func NewRecursiveBasePathFs(source afero.Fs, path string) *RecursiveBasePathFs {
+	return &RecursiveBasePathFs{
 		Fs:     afero.NewBasePathFs(source, path),
 		source: source,
 		path:   path,
 	}
 }
 
-// RealPath returns the real path to a file, "breaking out" of the BasePathFs.
-func (b *BasePathFs) RealPath(name string) (path string, err error) {
+// RealPath returns the real path to a file, "breaking out" of the RecursiveBasePathFs.
+func (b *RecursiveBasePathFs) RealPath(name string) (path string, err error) {
 	if err := validateBasePathName(name); err != nil {
 		return name, err
 	}
@@ -34,10 +34,10 @@ func (b *BasePathFs) RealPath(name string) (path string, err error) {
 	bpath := filepath.Clean(b.path)
 	path = filepath.Clean(filepath.Join(bpath, name))
 
-	if parentBasePathFs, ok := b.source.(*BasePathFs); ok {
-		return parentBasePathFs.RealPath(path)
-	} else if parentBasePathFs, ok := b.source.(*afero.BasePathFs); ok {
-		return parentBasePathFs.RealPath(path)
+	if parentRecursiveBasePathFs, ok := b.source.(*RecursiveBasePathFs); ok {
+		return parentRecursiveBasePathFs.RealPath(path)
+	} else if parentRecursiveBasePathFs, ok := b.source.(*afero.BasePathFs); ok {
+		return parentRecursiveBasePathFs.RealPath(path)
 	}
 
 	if !strings.HasPrefix(path, bpath) {
diff --git a/cmd/transpile/category.go b/cmd/transpile/category.go
index f2ab73a..f11198f 100644
--- a/cmd/transpile/category.go
+++ b/cmd/transpile/category.go
@@ -9,12 +9,16 @@ import (
 	"github.com/spf13/afero"
 )
 
+// NopReadCloser provides an io.ReadCloser which does nothing.
 type NopReadCloser struct {
 }
 
+// Read satisfies io.Reader.
 func (n NopReadCloser) Read(b []byte) (int, error) {
 	return 0, nil
 }
+
+// Close satisfies io.Closer.
 func (n NopReadCloser) Close() error {
 	return nil
 }
@@ -25,16 +29,16 @@ func (n NopReadCloser) Close() error {
 func NewFsCategory(fs afero.Fs) Category {
 	if info, err := fs.Stat("mkcategory"); (err == nil) && (info.Mode()&0100 != 0) {
 		return FsCommandCategory{fs: fs}
-	} else {
-		return FsCategory{fs: fs}
 	}
+	return FsCategory{fs: fs}
 }
 
+// FsCategory provides a category backed by a .md file.
 type FsCategory struct {
 	fs afero.Fs
 }
 
-// Category returns a list of puzzle values.
+// Inventory returns a list of point values for this category.
 func (c FsCategory) Inventory() ([]int, error) {
 	puzzleEntries, err := afero.ReadDir(c.fs, ".")
 	if err != nil {
@@ -56,14 +60,17 @@ func (c FsCategory) Inventory() ([]int, error) {
 	return puzzles, nil
 }
 
+// Puzzle returns a Puzzle structure for the given point value.
 func (c FsCategory) Puzzle(points int) (Puzzle, error) {
 	return NewFsPuzzle(c.fs, points).Puzzle()
 }
 
+// Open returns an io.ReadCloser for the given filename.
 func (c FsCategory) Open(points int, filename string) (io.ReadCloser, error) {
 	return NewFsPuzzle(c.fs, points).Open(filename)
 }
 
+// Answer check whether an answer is correct.
 func (c FsCategory) Answer(points int, answer string) bool {
 	// BUG(neale): FsCategory.Answer should probably always return false, to prevent you from running uncompiled puzzles with participants.
 	p, err := c.Puzzle(points)
@@ -78,22 +85,27 @@ func (c FsCategory) Answer(points int, answer string) bool {
 	return false
 }
 
+// FsCommandCategory provides a category backed by running an external command.
 type FsCommandCategory struct {
 	fs afero.Fs
 }
 
+// Inventory returns a list of point values for this category.
 func (c FsCommandCategory) Inventory() ([]int, error) {
 	return nil, fmt.Errorf("Not implemented")
 }
 
+// Puzzle returns a Puzzle structure for the given point value.
 func (c FsCommandCategory) Puzzle(points int) (Puzzle, error) {
 	return Puzzle{}, fmt.Errorf("Not implemented")
 }
 
+// Open returns an io.ReadCloser for the given filename.
 func (c FsCommandCategory) Open(points int, filename string) (io.ReadCloser, error) {
 	return NopReadCloser{}, fmt.Errorf("Not implemented")
 }
 
+// Answer check whether an answer is correct.
 func (c FsCommandCategory) Answer(points int, answer string) bool {
 	return false
 }
diff --git a/cmd/transpile/main.go b/cmd/transpile/main.go
index 9562746..b40bb7e 100644
--- a/cmd/transpile/main.go
+++ b/cmd/transpile/main.go
@@ -28,7 +28,7 @@ type Category interface {
 	Answer(points int, answer string) bool
 }
 
-// PuzzleDef contains everything about a puzzle.
+// Puzzle contains everything about a puzzle.
 type Puzzle struct {
 	Pre struct {
 		Authors       []string
@@ -155,8 +155,9 @@ func (t *T) Open() error {
 	return nil
 }
 
+// NewCategory returns a new Fs-backed category.
 func (t *T) NewCategory(name string) Category {
-	return NewFsCategory(NewBasePathFs(t.Fs, name))
+	return NewFsCategory(NewRecursiveBasePathFs(t.Fs, name))
 }
 
 func main() {
diff --git a/cmd/transpile/puzzle.go b/cmd/transpile/puzzle.go
index cf6d5a9..50c9d28 100644
--- a/cmd/transpile/puzzle.go
+++ b/cmd/transpile/puzzle.go
@@ -7,6 +7,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"log"
 	"net/mail"
 	"os/exec"
@@ -19,13 +20,36 @@ import (
 	"gopkg.in/yaml.v2"
 )
 
+// PuzzleProvider establishes the functionality required to provide one puzzle.
+type PuzzleProvider interface {
+	// Puzzle returns a Puzzle struct for the current puzzle.
+	Puzzle() (Puzzle, error)
+
+	// Open returns a newly-opened file.
+	Open(filename string) (io.ReadCloser, error)
+
+	// Answer returns whether the provided answer is correct.
+	Answer(answer string) bool
+}
+
 // NewFsPuzzle returns a new FsPuzzle for points.
-func NewFsPuzzle(fs afero.Fs, points int) *FsPuzzle {
-	fp := &FsPuzzle{
-		fs: NewBasePathFs(fs, strconv.Itoa(points)),
+func NewFsPuzzle(fs afero.Fs, points int) PuzzleProvider {
+	pfs := NewRecursiveBasePathFs(fs, strconv.Itoa(points))
+	if info, err := pfs.Stat("mkpuzzle"); (err == nil) && (info.Mode()&0100 != 0) {
+		if command, err := pfs.RealPath(info.Name()); err != nil {
+			log.Println("Unable to resolve full path to", info.Name(), pfs)
+		} else {
+			return FsCommandPuzzle{
+				fs:      pfs,
+				command: command,
+				timeout: 2 * time.Second,
+			}
+		}
 	}
 
-	return fp
+	return FsPuzzle{
+		fs: pfs,
+	}
 }
 
 // FsPuzzle is a single puzzle's directory.
@@ -162,29 +186,24 @@ func rfc822HeaderParser(r io.Reader) (Puzzle, error) {
 	return p, nil
 }
 
+// Answer checks whether the given answer is correct.
 func (fp FsPuzzle) Answer(answer string) bool {
 	return false
 }
 
+// FsCommandPuzzle provides an FsPuzzle backed by running a command.
 type FsCommandPuzzle struct {
-	fs afero.Fs
+	fs      afero.Fs
+	command string
+	timeout time.Duration
 }
 
+// Puzzle returns a Puzzle struct for the current puzzle.
 func (fp FsCommandPuzzle) Puzzle() (Puzzle, error) {
-	bfs, ok := fp.fs.(*BasePathFs)
-	if !ok {
-		return Puzzle{}, fmt.Errorf("Fs won't resolve real paths for %v", fp)
-	}
-	mkpuzzlePath, err := bfs.RealPath("mkpuzzle")
-	if err != nil {
-		return Puzzle{}, err
-	}
-	log.Print(mkpuzzlePath)
-
-	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	ctx, cancel := context.WithTimeout(context.Background(), fp.timeout)
 	defer cancel()
 
-	cmd := exec.CommandContext(ctx, mkpuzzlePath)
+	cmd := exec.CommandContext(ctx, fp.command)
 	stdout, err := cmd.Output()
 	if err != nil {
 		return Puzzle{}, err
@@ -200,10 +219,37 @@ func (fp FsCommandPuzzle) Puzzle() (Puzzle, error) {
 	return puzzle, nil
 }
 
+// Open returns a newly-opened file.
 func (fp FsCommandPuzzle) Open(filename string) (io.ReadCloser, error) {
-	return NopReadCloser{}, fmt.Errorf("Not implemented")
+	ctx, cancel := context.WithTimeout(context.Background(), fp.timeout)
+	defer cancel()
+
+	cmd := exec.CommandContext(ctx, fp.command, "-file", filename)
+	// BUG(neale): FsCommandPuzzle.Open() reads everything into memory, and will suck for large files.
+	out, err := cmd.Output()
+	if err != nil {
+		return NopReadCloser{}, err
+	}
+	buf := bytes.NewBuffer(out)
+
+	return ioutil.NopCloser(buf), nil
 }
 
+// Answer checks whether the given answer is correct.
 func (fp FsCommandPuzzle) Answer(answer string) bool {
+	ctx, cancel := context.WithTimeout(context.Background(), fp.timeout)
+	defer cancel()
+
+	cmd := exec.CommandContext(ctx, fp.command, "-answer", answer)
+	out, err := cmd.Output()
+	if err != nil {
+		log.Print("ERROR", err)
+		return false
+	}
+
+	switch strings.TrimSpace(string(out)) {
+	case "correct":
+		return true
+	}
 	return false
 }
diff --git a/cmd/transpile/puzzle_test.go b/cmd/transpile/puzzle_test.go
index 9b6a6d9..c0bd933 100644
--- a/cmd/transpile/puzzle_test.go
+++ b/cmd/transpile/puzzle_test.go
@@ -11,7 +11,7 @@ import (
 
 func TestPuzzle(t *testing.T) {
 	puzzleFs := newTestFs()
-	catFs := afero.NewBasePathFs(puzzleFs, "cat0")
+	catFs := NewRecursiveBasePathFs(puzzleFs, "cat0")
 
 	{
 		pd := NewFsPuzzle(catFs, 1)
@@ -70,10 +70,24 @@ func TestPuzzle(t *testing.T) {
 		t.Log(p)
 		t.Error("Wrong error for duplicate body:", err)
 	}
+
+	{
+		fs := afero.NewMemMapFs()
+		if err := afero.WriteFile(fs, "1/mkpuzzle", []byte("bleat"), 0755); err != nil {
+			t.Error(err)
+		}
+		p := NewFsPuzzle(fs, 1)
+		if _, ok := p.(FsCommandPuzzle); !ok {
+			t.Error("We didn't get an FsCommandPuzzle")
+		}
+		if _, err := p.Puzzle(); err == nil {
+			t.Error("We didn't get an error trying to run a command from a MemMapFs")
+		}
+	}
 }
 
 func TestFsPuzzle(t *testing.T) {
-	catFs := afero.NewBasePathFs(afero.NewOsFs(), "testdata")
+	catFs := NewRecursiveBasePathFs(afero.NewOsFs(), "testdata")
 
 	if _, err := NewFsPuzzle(catFs, 1).Puzzle(); err != nil {
 		t.Error(err)
@@ -88,16 +102,31 @@ func TestFsPuzzle(t *testing.T) {
 		t.Error(err)
 	}
 
-	if body, err := mkpuzzleDir.Open("moo.txt"); err != nil {
+	if r, err := mkpuzzleDir.Open("moo.txt"); err != nil {
 		t.Error(err)
 	} else {
-		defer body.Close()
+		defer r.Close()
 		buf := new(bytes.Buffer)
-		if _, err := io.Copy(buf, body); err != nil {
+		if _, err := io.Copy(buf, r); err != nil {
 			t.Error(err)
 		}
 		if buf.String() != "Moo.\n" {
-			t.Error("Wrong body")
+			t.Errorf("Wrong body: %#v", buf.String())
 		}
 	}
+
+	if r, err := mkpuzzleDir.Open("error"); err == nil {
+		r.Close()
+		t.Error("Error open didn't return error")
+	}
+
+	if !mkpuzzleDir.Answer("moo") {
+		t.Error("Right answer marked wrong")
+	}
+	if mkpuzzleDir.Answer("wrong") {
+		t.Error("Wrong answer marked correct")
+	}
+	if mkpuzzleDir.Answer("error") {
+		t.Error("Error answer marked correct")
+	}
 }
diff --git a/cmd/transpile/puzzlehandler.go b/cmd/transpile/puzzlehandler.go
deleted file mode 100644
index 06ab7d0..0000000
--- a/cmd/transpile/puzzlehandler.go
+++ /dev/null
@@ -1 +0,0 @@
-package main
diff --git a/cmd/transpile/testdata/3/mkpuzzle b/cmd/transpile/testdata/3/mkpuzzle
index d7d868b..df19db0 100755
--- a/cmd/transpile/testdata/3/mkpuzzle
+++ b/cmd/transpile/testdata/3/mkpuzzle
@@ -12,11 +12,33 @@ case $1 in
 }
 EOT
         ;;
-    moo.txt)
-        echo "Moo."
+    -file|--file)
+        case $2 in
+            moo.txt)
+                echo "Moo."
+                ;;
+            *)
+                echo "ERROR: no such file: $1" 1>&2
+                exit 1
+                ;;
+        esac
+        ;;
+    -answer|--answer)
+        case $2 in
+            moo)
+                echo "correct"
+                ;;
+            error)
+                echo "error" 1>&2
+                exit 1
+                ;;
+            *)
+                echo "incorrect"
+                ;;
+        esac
         ;;
     *)
-        echo "Error: no such file: $1" 1>&2
+        echo "ERROR: don't know what to do with $1" 1>&2
         exit 1
         ;;
 esac