diff --git a/engine/builtin_test.go b/engine/builtin_test.go
index d982ac4..b8a39e1 100644
--- a/engine/builtin_test.go
+++ b/engine/builtin_test.go
@@ -5,9 +5,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/mock"
-	orderedmap "github.com/wk8/go-ordered-map/v2"
 	"io"
 	"math"
 	"os"
@@ -16,6 +13,10 @@ import (
 	"strings"
 	"testing"
 	"unicode/utf8"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/mock"
+	orderedmap "github.com/wk8/go-ordered-map/v2"
 )
 
 type procedurePair orderedmap.Pair[procedureIndicator, procedure]
@@ -2709,8 +2710,8 @@ func TestAssertz(t *testing.T) {
 					args:    []Term{NewAtom("a")},
 				},
 				bytecode: bytecode{
-					{opcode: opGetConst, operand: NewAtom("a")},
-					{opcode: opExit},
+					{opcode: OpGetConst, operand: NewAtom("a")},
+					{opcode: OpExit},
 				},
 			},
 			{
@@ -2723,8 +2724,8 @@ func TestAssertz(t *testing.T) {
 					args:    []Term{NewAtom("b")},
 				},
 				bytecode: bytecode{
-					{opcode: opGetConst, operand: NewAtom("b")},
-					{opcode: opExit},
+					{opcode: OpGetConst, operand: NewAtom("b")},
+					{opcode: OpExit},
 				},
 			},
 		}}, vm.procedures.GetPair(procedureIndicator{
@@ -2839,8 +2840,8 @@ func TestAsserta(t *testing.T) {
 					args:    []Term{NewAtom("b")},
 				},
 				bytecode: bytecode{
-					{opcode: opGetConst, operand: NewAtom("b")},
-					{opcode: opExit},
+					{opcode: OpGetConst, operand: NewAtom("b")},
+					{opcode: OpExit},
 				},
 			},
 			{
@@ -2850,8 +2851,8 @@ func TestAsserta(t *testing.T) {
 					args:    []Term{NewAtom("a")},
 				},
 				bytecode: bytecode{
-					{opcode: opGetConst, operand: NewAtom("a")},
-					{opcode: opExit},
+					{opcode: OpGetConst, operand: NewAtom("a")},
+					{opcode: OpExit},
 				},
 			},
 		}}, vm.procedures.GetPair(procedureIndicator{name: NewAtom("foo"), arity: 1}).Value)
@@ -2902,11 +2903,11 @@ func TestAsserta(t *testing.T) {
 					},
 				},
 				bytecode: bytecode{
-					{opcode: opEnter},
-					{opcode: opPutConst, operand: NewAtom("a")},
-					{opcode: opCall, operand: procedureIndicator{name: NewAtom("p"), arity: 1}},
-					{opcode: opCut},
-					{opcode: opExit},
+					{opcode: OpEnter},
+					{opcode: OpPutConst, operand: NewAtom("a")},
+					{opcode: OpCall, operand: procedureIndicator{name: NewAtom("p"), arity: 1}},
+					{opcode: OpCut},
+					{opcode: OpExit},
 				},
 			},
 			{
@@ -2922,10 +2923,10 @@ func TestAsserta(t *testing.T) {
 					},
 				},
 				bytecode: bytecode{
-					{opcode: opEnter},
-					{opcode: opPutConst, operand: NewAtom("b")},
-					{opcode: opCall, operand: procedureIndicator{name: NewAtom("p"), arity: 1}},
-					{opcode: opExit},
+					{opcode: OpEnter},
+					{opcode: OpPutConst, operand: NewAtom("b")},
+					{opcode: OpCall, operand: procedureIndicator{name: NewAtom("p"), arity: 1}},
+					{opcode: OpExit},
 				},
 			},
 		}}, vm.procedures.GetPair(procedureIndicator{name: NewAtom("foo"), arity: 0}).Value)
diff --git a/engine/clause.go b/engine/clause.go
index 51e7e8f..e28916f 100644
--- a/engine/clause.go
+++ b/engine/clause.go
@@ -71,7 +71,7 @@ func compileClause(head Term, body Term, env *Env) (clause, error) {
 			return c, typeError(validTypeCallable, body, env)
 		}
 	}
-	c.bytecode = append(c.bytecode, instruction{opcode: opExit})
+	c.bytecode = append(c.bytecode, instruction{opcode: OpExit})
 	return c, nil
 }
 
@@ -88,7 +88,7 @@ func (c *clause) compileHead(head Term, env *Env) {
 }
 
 func (c *clause) compileBody(body Term, env *Env) error {
-	c.bytecode = append(c.bytecode, instruction{opcode: opEnter})
+	c.bytecode = append(c.bytecode, instruction{opcode: OpEnter})
 	iter := seqIterator{Seq: body, Env: env}
 	for iter.Next() {
 		if err := c.compilePred(iter.Current(), env); err != nil {
@@ -107,16 +107,16 @@ func (c *clause) compilePred(p Term, env *Env) error {
 	case Atom:
 		switch p {
 		case atomCut:
-			c.bytecode = append(c.bytecode, instruction{opcode: opCut})
+			c.bytecode = append(c.bytecode, instruction{opcode: OpCut})
 			return nil
 		}
-		c.bytecode = append(c.bytecode, instruction{opcode: opCall, operand: procedureIndicator{name: p, arity: 0}})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpCall, operand: procedureIndicator{name: p, arity: 0}})
 		return nil
 	case Compound:
 		for i := 0; i < p.Arity(); i++ {
 			c.compileBodyArg(p.Arg(i), env)
 		}
-		c.bytecode = append(c.bytecode, instruction{opcode: opCall, operand: procedureIndicator{name: p.Functor(), arity: Integer(p.Arity())}})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpCall, operand: procedureIndicator{name: p.Functor(), arity: Integer(p.Arity())}})
 		return nil
 	default:
 		return errNotCallable
@@ -126,67 +126,67 @@ func (c *clause) compilePred(p Term, env *Env) error {
 func (c *clause) compileHeadArg(a Term, env *Env) {
 	switch a := env.Resolve(a).(type) {
 	case Variable:
-		c.bytecode = append(c.bytecode, instruction{opcode: opGetVar, operand: c.varOffset(a)})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpGetVar, operand: c.varOffset(a)})
 	case charList, codeList: // Treat them as if they're atomic.
-		c.bytecode = append(c.bytecode, instruction{opcode: opGetConst, operand: a})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpGetConst, operand: a})
 	case list:
-		c.bytecode = append(c.bytecode, instruction{opcode: opGetList, operand: Integer(len(a))})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpGetList, operand: Integer(len(a))})
 		for _, arg := range a {
 			c.compileHeadArg(arg, env)
 		}
-		c.bytecode = append(c.bytecode, instruction{opcode: opPop})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpPop})
 	case *partial:
 		prefix := a.Compound.(list)
-		c.bytecode = append(c.bytecode, instruction{opcode: opGetPartial, operand: Integer(len(prefix))})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpGetPartial, operand: Integer(len(prefix))})
 		c.compileHeadArg(*a.tail, env)
 		for _, arg := range prefix {
 			c.compileHeadArg(arg, env)
 		}
-		c.bytecode = append(c.bytecode, instruction{opcode: opPop})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpPop})
 	case Compound:
-		c.bytecode = append(c.bytecode, instruction{opcode: opGetFunctor, operand: procedureIndicator{name: a.Functor(), arity: Integer(a.Arity())}})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpGetFunctor, operand: procedureIndicator{name: a.Functor(), arity: Integer(a.Arity())}})
 		for i := 0; i < a.Arity(); i++ {
 			c.compileHeadArg(a.Arg(i), env)
 		}
-		c.bytecode = append(c.bytecode, instruction{opcode: opPop})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpPop})
 	default:
-		c.bytecode = append(c.bytecode, instruction{opcode: opGetConst, operand: a})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpGetConst, operand: a})
 	}
 }
 
 func (c *clause) compileBodyArg(a Term, env *Env) {
 	switch a := env.Resolve(a).(type) {
 	case Variable:
-		c.bytecode = append(c.bytecode, instruction{opcode: opPutVar, operand: c.varOffset(a)})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpPutVar, operand: c.varOffset(a)})
 	case charList, codeList: // Treat them as if they're atomic.
-		c.bytecode = append(c.bytecode, instruction{opcode: opPutConst, operand: a})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpPutConst, operand: a})
 	case list:
-		c.bytecode = append(c.bytecode, instruction{opcode: opPutList, operand: Integer(len(a))})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpPutList, operand: Integer(len(a))})
 		for _, arg := range a {
 			c.compileBodyArg(arg, env)
 		}
-		c.bytecode = append(c.bytecode, instruction{opcode: opPop})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpPop})
 	case *partial:
 		var l int
 		iter := ListIterator{List: a.Compound}
 		for iter.Next() {
 			l++
 		}
-		c.bytecode = append(c.bytecode, instruction{opcode: opPutPartial, operand: Integer(l)})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpPutPartial, operand: Integer(l)})
 		c.compileBodyArg(*a.tail, env)
 		iter = ListIterator{List: a.Compound}
 		for iter.Next() {
 			c.compileBodyArg(iter.Current(), env)
 		}
-		c.bytecode = append(c.bytecode, instruction{opcode: opPop})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpPop})
 	case Compound:
-		c.bytecode = append(c.bytecode, instruction{opcode: opPutFunctor, operand: procedureIndicator{name: a.Functor(), arity: Integer(a.Arity())}})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpPutFunctor, operand: procedureIndicator{name: a.Functor(), arity: Integer(a.Arity())}})
 		for i := 0; i < a.Arity(); i++ {
 			c.compileBodyArg(a.Arg(i), env)
 		}
-		c.bytecode = append(c.bytecode, instruction{opcode: opPop})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpPop})
 	default:
-		c.bytecode = append(c.bytecode, instruction{opcode: opPutConst, operand: a})
+		c.bytecode = append(c.bytecode, instruction{opcode: OpPutConst, operand: a})
 	}
 }
 
diff --git a/engine/text_test.go b/engine/text_test.go
index 5f2da3b..0fa5139 100644
--- a/engine/text_test.go
+++ b/engine/text_test.go
@@ -4,11 +4,12 @@ import (
 	"context"
 	"embed"
 	"errors"
-	orderedmap "github.com/wk8/go-ordered-map/v2"
 	"io"
 	"io/fs"
 	"testing"
 
+	orderedmap "github.com/wk8/go-ordered-map/v2"
+
 	"github.com/stretchr/testify/assert"
 )
 
@@ -44,8 +45,8 @@ foo(a).
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("a")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("a")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("a")},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -62,8 +63,8 @@ foo(a).
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("c")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("c")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("c")},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -82,16 +83,16 @@ foo(b).
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("a")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("a")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("a")},
+								{opcode: OpExit},
 							},
 						},
 						{
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("b")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("b")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("b")},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -111,8 +112,8 @@ bar(X, "abc", [a, b], [a, b|Y], f(a)) :- X, !, foo(X, "abc", [a, b], [a, b|Y], f
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("c")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("c")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("c")},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -126,9 +127,9 @@ bar(X, "abc", [a, b], [a, b|Y], f(a)) :- X, !, foo(X, "abc", [a, b], [a, b|Y], f
 							pi:  procedureIndicator{name: NewAtom("bar"), arity: 0},
 							raw: atomIf.Apply(NewAtom("bar"), atomTrue),
 							bytecode: bytecode{
-								{opcode: opEnter},
-								{opcode: opCall, operand: procedureIndicator{name: atomTrue, arity: 0}},
-								{opcode: opExit},
+								{opcode: OpEnter},
+								{opcode: OpCall, operand: procedureIndicator{name: atomTrue, arity: 0}},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -150,40 +151,40 @@ bar(X, "abc", [a, b], [a, b|Y], f(a)) :- X, !, foo(X, "abc", [a, b], [a, b|Y], f
 							),
 							vars: []Variable{lastVariable() + 1, lastVariable() + 2},
 							bytecode: bytecode{
-								{opcode: opGetVar, operand: Integer(0)},
-								{opcode: opGetConst, operand: charList("abc")},
-								{opcode: opGetList, operand: Integer(2)},
-								{opcode: opGetConst, operand: NewAtom("a")},
-								{opcode: opGetConst, operand: NewAtom("b")},
-								{opcode: opPop},
-								{opcode: opGetPartial, operand: Integer(2)},
-								{opcode: opGetVar, operand: Integer(1)},
-								{opcode: opGetConst, operand: NewAtom("a")},
-								{opcode: opGetConst, operand: NewAtom("b")},
-								{opcode: opPop},
-								{opcode: opGetFunctor, operand: procedureIndicator{name: NewAtom("f"), arity: 1}},
-								{opcode: opGetConst, operand: NewAtom("a")},
-								{opcode: opPop},
-								{opcode: opEnter},
-								{opcode: opPutVar, operand: Integer(0)},
-								{opcode: opCall, operand: procedureIndicator{name: atomCall, arity: 1}},
-								{opcode: opCut},
-								{opcode: opPutVar, operand: Integer(0)},
-								{opcode: opPutConst, operand: charList("abc")},
-								{opcode: opPutList, operand: Integer(2)},
-								{opcode: opPutConst, operand: NewAtom("a")},
-								{opcode: opPutConst, operand: NewAtom("b")},
-								{opcode: opPop},
-								{opcode: opPutPartial, operand: Integer(2)},
-								{opcode: opPutVar, operand: Integer(1)},
-								{opcode: opPutConst, operand: NewAtom("a")},
-								{opcode: opPutConst, operand: NewAtom("b")},
-								{opcode: opPop},
-								{opcode: opPutFunctor, operand: procedureIndicator{name: NewAtom("f"), arity: 1}},
-								{opcode: opPutConst, operand: NewAtom("a")},
-								{opcode: opPop},
-								{opcode: opCall, operand: procedureIndicator{name: NewAtom("foo"), arity: 5}},
-								{opcode: opExit},
+								{opcode: OpGetVar, operand: Integer(0)},
+								{opcode: OpGetConst, operand: charList("abc")},
+								{opcode: OpGetList, operand: Integer(2)},
+								{opcode: OpGetConst, operand: NewAtom("a")},
+								{opcode: OpGetConst, operand: NewAtom("b")},
+								{opcode: OpPop},
+								{opcode: OpGetPartial, operand: Integer(2)},
+								{opcode: OpGetVar, operand: Integer(1)},
+								{opcode: OpGetConst, operand: NewAtom("a")},
+								{opcode: OpGetConst, operand: NewAtom("b")},
+								{opcode: OpPop},
+								{opcode: OpGetFunctor, operand: procedureIndicator{name: NewAtom("f"), arity: 1}},
+								{opcode: OpGetConst, operand: NewAtom("a")},
+								{opcode: OpPop},
+								{opcode: OpEnter},
+								{opcode: OpPutVar, operand: Integer(0)},
+								{opcode: OpCall, operand: procedureIndicator{name: atomCall, arity: 1}},
+								{opcode: OpCut},
+								{opcode: OpPutVar, operand: Integer(0)},
+								{opcode: OpPutConst, operand: charList("abc")},
+								{opcode: OpPutList, operand: Integer(2)},
+								{opcode: OpPutConst, operand: NewAtom("a")},
+								{opcode: OpPutConst, operand: NewAtom("b")},
+								{opcode: OpPop},
+								{opcode: OpPutPartial, operand: Integer(2)},
+								{opcode: OpPutVar, operand: Integer(1)},
+								{opcode: OpPutConst, operand: NewAtom("a")},
+								{opcode: OpPutConst, operand: NewAtom("b")},
+								{opcode: OpPop},
+								{opcode: OpPutFunctor, operand: procedureIndicator{name: NewAtom("f"), arity: 1}},
+								{opcode: OpPutConst, operand: NewAtom("a")},
+								{opcode: OpPop},
+								{opcode: OpCall, operand: procedureIndicator{name: NewAtom("foo"), arity: 5}},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -205,16 +206,16 @@ foo(b).
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("a")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("a")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("a")},
+								{opcode: OpExit},
 							},
 						},
 						{
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("b")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("b")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("b")},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -235,24 +236,24 @@ foo(b).
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("c")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("c")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("c")},
+								{opcode: OpExit},
 							},
 						},
 						{
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("a")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("a")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("a")},
+								{opcode: OpExit},
 							},
 						},
 						{
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("b")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("b")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("b")},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -274,16 +275,16 @@ foo(b).
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("a")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("a")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("a")},
+								{opcode: OpExit},
 							},
 						},
 						{
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("b")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("b")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("b")},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -297,8 +298,8 @@ foo(b).
 							pi:  procedureIndicator{name: NewAtom("bar"), arity: 1},
 							raw: &compound{functor: NewAtom("bar"), args: []Term{NewAtom("a")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("a")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("a")},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -317,8 +318,8 @@ foo(b).
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("c")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("c")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("c")},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -332,7 +333,7 @@ foo(b).
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 0},
 							raw: NewAtom("foo"),
 							bytecode: bytecode{
-								{opcode: opExit},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -351,8 +352,8 @@ foo(b).
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("c")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("c")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("c")},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -366,7 +367,7 @@ foo(b).
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 0},
 							raw: NewAtom("foo"),
 							bytecode: bytecode{
-								{opcode: opExit},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -385,8 +386,8 @@ foo(b).
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("c")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("c")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("c")},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -405,8 +406,8 @@ foo(b).
 							pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 							raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("c")}},
 							bytecode: bytecode{
-								{opcode: opGetConst, operand: NewAtom("c")},
-								{opcode: opExit},
+								{opcode: OpGetConst, operand: NewAtom("c")},
+								{opcode: OpExit},
 							},
 						},
 					},
@@ -500,8 +501,8 @@ bar(b).
 								pi:  procedureIndicator{name: NewAtom("foo"), arity: 1},
 								raw: &compound{functor: NewAtom("foo"), args: []Term{NewAtom("c")}},
 								bytecode: bytecode{
-									{opcode: opGetConst, operand: NewAtom("c")},
-									{opcode: opExit},
+									{opcode: OpGetConst, operand: NewAtom("c")},
+									{opcode: OpExit},
 								},
 							},
 						},
diff --git a/engine/vm.go b/engine/vm.go
index acfae9d..b797b5e 100644
--- a/engine/vm.go
+++ b/engine/vm.go
@@ -1,42 +1,102 @@
 package engine
 
 import (
+	"bytes"
 	"context"
 	"fmt"
-	orderedmap "github.com/wk8/go-ordered-map/v2"
 	"io"
 	"io/fs"
 	"strings"
+
+	orderedmap "github.com/wk8/go-ordered-map/v2"
 )
 
+// HookFunc is a type for a hook function that is triggered before the VM executes a specific instruction.
+// If the hook function returns an error, the VM halts execution and returns the error.
+type HookFunc func(opcode Opcode, operand Term, env *Env) error
+
+// DebugHook is a hook function that prints the current instruction and its operand (if any).
+func DebugHook(opcode Opcode, operand Term, env *Env) error {
+	var buf bytes.Buffer
+
+	buf.WriteString(opcode.String())
+
+	if operand != nil {
+		buf.WriteRune('(')
+		_ = operand.WriteTerm(&buf, &defaultWriteOptions, nil)
+		buf.WriteRune(')')
+	}
+	fmt.Println(buf.String())
+
+	return nil
+}
+
+// CompositeHook returns a hook function that chains multiple hooks together.
+// The hooks are executed sequentially, and if any hook returns an error, the execution stops.
+func CompositeHook(fs ...HookFunc) HookFunc {
+	return func(opcode Opcode, operand Term, env *Env) error {
+		for _, f := range fs {
+			if err := f(opcode, operand, env); err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+}
+
 type bytecode []instruction
 
 type instruction struct {
-	opcode  opcode
+	opcode  Opcode
 	operand Term
 }
 
-type opcode byte
+type Opcode byte
 
 const (
-	opEnter opcode = iota
-	opCall
-	opExit
-	opGetConst
-	opPutConst
-	opGetVar
-	opPutVar
-	opGetFunctor
-	opPutFunctor
-	opPop
-
-	opCut
-	opGetList
-	opPutList
-	opGetPartial
-	opPutPartial
+	OpEnter Opcode = iota
+	OpCall
+	OpExit
+	OpGetConst
+	OpPutConst
+	OpGetVar
+	OpPutVar
+	OpGetFunctor
+	OpPutFunctor
+	OpPop
+
+	OpCut
+	OpGetList
+	OpPutList
+	OpGetPartial
+	OpPutPartial
 )
 
+func (op Opcode) String() string {
+	opcodeStrings := [...]string{
+		OpEnter:      "enter",
+		OpCall:       "call",
+		OpExit:       "exit",
+		OpGetConst:   "get_const",
+		OpPutConst:   "put_const",
+		OpGetVar:     "get_var",
+		OpPutVar:     "put_var",
+		OpGetFunctor: "get_functor",
+		OpPutFunctor: "put_functor",
+		OpPop:        "pop",
+		OpCut:        "cut",
+		OpGetList:    "get_list",
+		OpPutList:    "put_list",
+		OpGetPartial: "get_partial",
+		OpPutPartial: "put_partial",
+	}
+
+	if int(op) < 0 || int(op) >= len(opcodeStrings) {
+		return fmt.Sprintf("(%d)", op)
+	}
+	return opcodeStrings[op]
+}
+
 // Success is a continuation that leads to true.
 func Success(*Env) *Promise {
 	return Bool(true)
@@ -73,6 +133,9 @@ type VM struct {
 	// Limits
 	maxVariables uint64
 
+	// Hook
+	hook HookFunc
+
 	// Misc
 	debug bool
 }
@@ -181,20 +244,26 @@ func (vm *VM) exec(pc bytecode, vars []Variable, cont Cont, args []Term, astack
 	)
 	for ok {
 		op, pc = pc[0], pc[1:]
+		if vm.hook != nil {
+			if err := vm.hook(op.opcode, op.operand, env); err != nil {
+				return Error(err)
+			}
+		}
+
 		switch opcode, operand := op.opcode, op.operand; opcode {
-		case opGetConst:
+		case OpGetConst:
 			arg, args = args[0], args[1:]
 			env, ok = env.Unify(arg, operand)
-		case opPutConst:
+		case OpPutConst:
 			args = append(args, operand)
-		case opGetVar:
+		case OpGetVar:
 			v := vars[operand.(Integer)]
 			arg, args = args[0], args[1:]
 			env, ok = env.Unify(arg, v)
-		case opPutVar:
+		case OpPutVar:
 			v := vars[operand.(Integer)]
 			args = append(args, v)
-		case opGetFunctor:
+		case OpGetFunctor:
 			pi := operand.(procedureIndicator)
 			arg, astack = env.Resolve(args[0]), append(astack, args[1:])
 			args = make([]Term, int(pi.arity))
@@ -202,29 +271,29 @@ func (vm *VM) exec(pc bytecode, vars []Variable, cont Cont, args []Term, astack
 				args[i] = NewVariable()
 			}
 			env, ok = env.Unify(arg, pi.name.Apply(args...))
-		case opPutFunctor:
+		case OpPutFunctor:
 			pi := operand.(procedureIndicator)
 			vs := make([]Term, int(pi.arity))
 			arg = pi.name.Apply(vs...)
 			args = append(args, arg)
 			astack = append(astack, args)
 			args = vs[:0]
-		case opPop:
+		case OpPop:
 			args, astack = astack[len(astack)-1], astack[:len(astack)-1]
-		case opEnter:
+		case OpEnter:
 			break
-		case opCall:
+		case OpCall:
 			pi := operand.(procedureIndicator)
 			return vm.Arrive(pi.name, args, func(env *Env) *Promise {
 				return vm.exec(pc, vars, cont, nil, nil, env, cutParent)
 			}, env)
-		case opExit:
+		case OpExit:
 			return cont(env)
-		case opCut:
+		case OpCut:
 			return cut(cutParent, func(context.Context) *Promise {
 				return vm.exec(pc, vars, cont, args, astack, env, cutParent)
 			})
-		case opGetList:
+		case OpGetList:
 			l := operand.(Integer)
 			arg, astack = args[0], append(astack, args[1:])
 			args = make([]Term, int(l))
@@ -232,14 +301,14 @@ func (vm *VM) exec(pc bytecode, vars []Variable, cont Cont, args []Term, astack
 				args[i] = NewVariable()
 			}
 			env, ok = env.Unify(arg, list(args))
-		case opPutList:
+		case OpPutList:
 			l := operand.(Integer)
 			vs := make([]Term, int(l))
 			arg = list(vs)
 			args = append(args, arg)
 			astack = append(astack, args)
 			args = vs[:0]
-		case opGetPartial:
+		case OpGetPartial:
 			l := operand.(Integer)
 			arg, astack = args[0], append(astack, args[1:])
 			args = make([]Term, int(l+1))
@@ -247,7 +316,7 @@ func (vm *VM) exec(pc bytecode, vars []Variable, cont Cont, args []Term, astack
 				args[i] = NewVariable()
 			}
 			env, ok = env.Unify(arg, PartialList(args[0], args[1:]...))
-		case opPutPartial:
+		case OpPutPartial:
 			l := operand.(Integer)
 			vs := make([]Term, int(l+1))
 			arg = &partial{
@@ -286,6 +355,16 @@ func (vm *VM) SetMaxVariables(n uint64) {
 	maxVariables = n
 }
 
+// InstallHook sets the given hook function in the VM.
+func (vm *VM) InstallHook(f HookFunc) {
+	vm.hook = f
+}
+
+// ClearHook removes the installed hook function from the VM.
+func (vm *VM) ClearHook() {
+	vm.hook = nil
+}
+
 // ResetEnv is used to reset all global variable
 func (vm *VM) ResetEnv() {
 	varCounter.count = 0