diff --git a/go/router/router.go b/go/router/router.go
index eedea83..4ec7cf3 100644
--- a/go/router/router.go
+++ b/go/router/router.go
@@ -7,20 +7,17 @@ import (
 type Router struct {
 	selectors []string
 	symbols map[string]string
-	navigable bool
 }
 
 func NewRouter() Router {
 	return Router{
 		symbols: make(map[string]string),
-		navigable: true,
 	}
 }
 
 func NewStaticRouter(symbol string) Router {
 	return Router{
 		symbols: map[string]string{"_": symbol},
-		navigable: false,
 	}
 }
 
@@ -32,6 +29,9 @@ func(r *Router) Add(selector string, symbol string) error {
 	if (l > 255) {
 		return fmt.Errorf("selector too long (is %v, max 255)", l)
 	}
+	if selector[0] == '_' {
+		return fmt.Errorf("Invalid selector prefix '_'")
+	}
 	l = len(symbol)
 	if (l > 255) {
 		return fmt.Errorf("symbol too long (is %v, max 255)", l)
diff --git a/go/state/state.go b/go/state/state.go
index bf84f23..3ece243 100644
--- a/go/state/state.go
+++ b/go/state/state.go
@@ -34,6 +34,14 @@ func NewState(bitSize uint64) State {
 	return st
 }
 
+func(st State) Where() string {
+	if len(st.ExecPath) == 0 {
+		return ""
+	}
+	l := len(st.ExecPath)
+	return st.ExecPath[l-1]
+}
+
 func(st State) WithCacheSize(cacheSize uint32) State {
 	st.CacheSize = cacheSize
 	return st
diff --git a/go/vm/opcodes.go b/go/vm/opcodes.go
index 23aed1c..3960338 100644
--- a/go/vm/opcodes.go
+++ b/go/vm/opcodes.go
@@ -1,13 +1,35 @@
 package vm
 
+import (
+	"encoding/binary"
+)
 const VERSION = 0
 
 const (
-	CATCH = iota
+	BACK = iota
+	CATCH
 	CROAK
 	LOAD
 	RELOAD
 	MAP
 	SINK
+	MOVE
 	_MAX
 )
+
+func NewLine(instructionList []byte, instruction uint16, args []string, post []byte, szPost []uint8) []byte {
+	b := []byte{0x00, 0x00}
+	binary.BigEndian.PutUint16(b, instruction)
+	for _, arg := range args {
+		b = append(b, uint8(len(arg)))
+		b = append(b, []byte(arg)...)
+	}
+	if post != nil {
+		b = append(b, uint8(len(post)))
+		b = append(b, post...)
+	}
+	if szPost != nil {
+		b = append(b, szPost...)
+	}
+	return append(instructionList, b...)
+}
diff --git a/go/vm/vm.go b/go/vm/vm.go
index 76e9f3a..d61c925 100644
--- a/go/vm/vm.go
+++ b/go/vm/vm.go
@@ -13,19 +13,39 @@ import (
 
 type Runner func(instruction []byte, st state.State, rs resource.Fetcher, ctx context.Context) (state.State, []byte, error)
 
+func argFromBytes(input []byte) (string, []byte, error) {
+	if len(input) == 0 {
+		return "", input, fmt.Errorf("zero length input")
+	}
+	sz := input[0]
+	out := input[1:1+sz]
+	return string(out), input[1+sz:], nil
+}
+
 func Apply(input []byte, instruction []byte, st state.State, rs resource.Fetcher, ctx context.Context) (state.State, []byte, error) {
 	var err error
+
+	arg, input, err := argFromBytes(input)
+	if err != nil {
+		return st, input, err
+	}
+
+	rt := router.FromBytes(input)
+	sym := rt.Get(arg)
+	if sym == "" {
+		sym = rt.Default()
+		st.PutArg(arg)
+	} 
+	if sym == "" {
+		instruction = NewLine([]byte{}, MOVE, []string{"_catch"}, nil , nil)
+	} else {
+		instruction = NewLine(instruction, MOVE, []string{sym}, nil, nil)
+	}
+
 	st, instruction, err = Run(instruction, st, rs, ctx)
 	if err != nil {
 		return st, instruction, err
 	}
-	rt := router.FromBytes(instruction)
-	
-	sym := rt.Get(string(input))
-	if sym == "" {
-		sym = rt.Default()
-		st.PutArg(string(input))
-	}
 	return st, instruction, nil
 }
 
@@ -56,6 +76,9 @@ func Run(instruction []byte, st state.State, rs resource.Fetcher, ctx context.Co
 		case SINK:
 			st, instruction, err = RunSink(instruction[2:], st, rs, ctx)
 			break
+		case MOVE:
+			st, instruction, err = RunMove(instruction[2:], st, rs, ctx)
+			break
 		default:
 			err = fmt.Errorf("Unhandled state: %v", op)
 		}
@@ -88,8 +111,9 @@ func RunCatch(instruction []byte, st state.State, rs resource.Fetcher, ctx conte
 	if err != nil {
 		return st, instruction, err
 	}
+	_ = tail
 	st.Add(head, r, uint32(len(r)))
-	return st, tail, nil
+	return st, []byte{}, nil
 }
 
 func RunCroak(instruction []byte, st state.State, rs resource.Fetcher, ctx context.Context) (state.State, []byte, error) {
@@ -98,8 +122,9 @@ func RunCroak(instruction []byte, st state.State, rs resource.Fetcher, ctx conte
 		return st, instruction, err
 	}
 	_ = head
+	_ = tail
 	st.Reset()
-	return st, tail, nil
+	return st, []byte{}, nil
 }
 
 func RunLoad(instruction []byte, st state.State, rs resource.Fetcher, ctx context.Context) (state.State, []byte, error) {
@@ -135,7 +160,12 @@ func RunReload(instruction []byte, st state.State, rs resource.Fetcher, ctx cont
 }
 
 func RunMove(instruction []byte, st state.State, rs resource.Fetcher, ctx context.Context) (state.State, []byte, error) {
-	return st, nil, nil
+	head, tail, err := instructionSplit(instruction)
+	if err != nil {
+		return st, instruction, err
+	}
+	st.Down(head)
+	return st, tail, nil
 }
 
 func refresh(key string, sym []byte, rs resource.Fetcher, ctx context.Context) (string, error) {
diff --git a/go/vm/vm_test.go b/go/vm/vm_test.go
index 0c1c855..4de5253 100644
--- a/go/vm/vm_test.go
+++ b/go/vm/vm_test.go
@@ -3,13 +3,13 @@ package vm
 import (
 	"bytes"
 	"context"
-	"encoding/binary"
 	"fmt"
 	"log"
 	"testing"
 	"text/template"
 	
 	"git.defalsify.org/festive/resource"
+	"git.defalsify.org/festive/router"
 	"git.defalsify.org/festive/state"
 )
 
@@ -159,8 +159,8 @@ func TestRunMultiple(t *testing.T) {
 	st := state.NewState(5)
 	rs := TestResource{}
 	b := []byte{}
-	b = NewTestOp(b, LOAD, []string{"one"}, nil, []uint8{0})
-	b = NewTestOp(b, LOAD, []string{"two"}, nil, []uint8{42})
+	b = NewLine(b, LOAD, []string{"one"}, nil, []uint8{0})
+	b = NewLine(b, LOAD, []string{"two"}, nil, []uint8{42})
 	st, _, err := Run(b, st, &rs, context.TODO())
 	if err != nil {
 		t.Error(err)
@@ -171,8 +171,8 @@ func TestRunReload(t *testing.T) {
 	st := state.NewState(5)
 	rs := TestResource{}
 	b := []byte{}
-	b = NewTestOp(b, LOAD, []string{"dyn"}, nil, []uint8{0})
-	b = NewTestOp(b, MAP, []string{"dyn"}, nil, nil)
+	b = NewLine(b, LOAD, []string{"dyn"}, nil, []uint8{0})
+	b = NewLine(b, MAP, []string{"dyn"}, nil, nil)
 	st, _, err := Run(b, st, &rs, context.TODO())
 	if err != nil {
 		t.Error(err)
@@ -186,7 +186,7 @@ func TestRunReload(t *testing.T) {
 	}
 	dynVal = "baz"
 	b = []byte{}
-	b = NewTestOp(b, RELOAD, []string{"dyn"}, nil, nil)
+	b = NewLine(b, RELOAD, []string{"dyn"}, nil, nil)
 	st, _, err = Run(b, st, &rs, context.TODO())
 	if err != nil {
 		t.Error(err)
@@ -202,19 +202,50 @@ func TestRunReload(t *testing.T) {
 
 }
 
-func NewTestOp(instructionList []byte, instruction uint16, args []string, post []byte, szPost []uint8) []byte {
-	b := []byte{0x00, 0x00}
-	binary.BigEndian.PutUint16(b, instruction)
-	for _, arg := range args {
-		b = append(b, uint8(len(arg)))
-		b = append(b, []byte(arg)...)
+func TestRunArg(t *testing.T) {
+	st := state.NewState(5)
+	rt := router.NewRouter()
+	rt.Add("foo", "bar")
+	rt.Add("baz", "xyzzy")
+	b := []byte{0x03}
+	b = append(b, []byte("baz")...)
+	b = append(b, rt.ToBytes()...)
+	var err error
+	st, b, err = Apply(b, []byte{}, st, nil, context.TODO())
+	if err != nil {
+		t.Error(err)	
 	}
-	if post != nil {
-		b = append(b, uint8(len(post)))
-		b = append(b, post...)
+	l := len(b)
+	if l != 0 {
+		t.Errorf("expected empty remainder, got length %v: %v", l, b)
 	}
-	if szPost != nil {
-		b = append(b, szPost...)
+	r := st.Where()
+	if r != "xyzzy" {
+		t.Errorf("expected where-state baz, got %v", r)
 	}
-	return append(instructionList, b...)
 }
+
+func TestRunArgInvalid(t *testing.T) {
+	st := state.NewState(5)
+	rt := router.NewRouter()
+	rt.Add("foo", "bar")
+	rt.Add("baz", "xyzzy")
+	b := []byte{0x03}
+	b = append(b, []byte("bar")...)
+	b = append(b, rt.ToBytes()...)
+	var err error
+	st, b, err = Apply(b, []byte{}, st, nil, context.TODO())
+	if err != nil {
+		t.Error(err)	
+	}
+	l := len(b)
+	if l != 0 {
+		t.Errorf("expected empty remainder, got length %v: %v", l, b)
+	}
+	r := st.Where()
+	if r != "_catch" {
+		t.Errorf("expected where-state _catch, got %v", r)
+	}
+
+}
+