diff --git a/cmd/lambda/lambda.go b/cmd/lambda/lambda.go
index 2089cb5..eade9ff 100644
--- a/cmd/lambda/lambda.go
+++ b/cmd/lambda/lambda.go
@@ -6,9 +6,7 @@ import (
 	"git.maximhutz.com/max/lambda/internal/cli"
 	"git.maximhutz.com/max/lambda/internal/config"
 	"git.maximhutz.com/max/lambda/internal/engine"
-	"git.maximhutz.com/max/lambda/internal/explanation"
-	"git.maximhutz.com/max/lambda/internal/performance"
-	"git.maximhutz.com/max/lambda/internal/statistics"
+	"git.maximhutz.com/max/lambda/internal/plugins"
 	"git.maximhutz.com/max/lambda/pkg/convert"
 	"git.maximhutz.com/max/lambda/pkg/lambda"
 	"git.maximhutz.com/max/lambda/pkg/saccharine"
@@ -42,30 +40,23 @@ func main() {
 	// If the user selected to track CPU performance, attach a profiler to the
 	// process.
 	if options.Profile != "" {
-		profiler := performance.Track(options.Profile)
-		process.On("start", profiler.Start)
-		process.On("end", profiler.End)
+		plugins.NewPerformance(options.Profile, process)
 	}
 
 	// If the user selected to produce a step-by-step explanation, attach an
 	// observer here.
 	if options.Explanation {
-		explanation.Track(process)
+		plugins.NewExplanation(process)
 	}
 
 	// If the user opted to track statistics, attach a tracker here, too.
 	if options.Statistics {
-		statistics := statistics.Track()
-		process.On("start", statistics.Start)
-		process.On("step", statistics.Step)
-		process.On("end", statistics.End)
+		plugins.NewStatistics(process)
 	}
 
 	// If the user selected for verbose debug logs, attach a reduction tracker.
 	if options.Verbose {
-		process.On("step", func() {
-			logger.Info("reduction", "tree", lambda.Stringify(compiled))
-		})
+		plugins.NewLogs(logger, process)
 	}
 
 	process.Run()
diff --git a/internal/engine/engine.go b/internal/engine/engine.go
index 0add767..49102ae 100644
--- a/internal/engine/engine.go
+++ b/internal/engine/engine.go
@@ -12,7 +12,7 @@ import (
 type Engine struct {
 	Config     *config.Config
 	Expression *lambda.Expression
-	emitter.Emitter
+	emitter.BaseEmitter[Event]
 }
 
 // Create a new engine, given an unreduced λ-expression.
@@ -22,11 +22,11 @@ func New(config *config.Config, expression *lambda.Expression) *Engine {
 
 // Begin the reduction process.
 func (e Engine) Run() {
-	e.Emit("start")
+	e.Emit(StartEvent)
 
 	lambda.ReduceAll(e.Expression, func() {
-		e.Emit("step")
+		e.Emit(StepEvent)
 	})
 
-	e.Emit("end")
+	e.Emit(StopEvent)
 }
diff --git a/internal/engine/events.go b/internal/engine/events.go
new file mode 100644
index 0000000..995ebea
--- /dev/null
+++ b/internal/engine/events.go
@@ -0,0 +1,9 @@
+package engine
+
+type Event int
+
+const (
+	StartEvent Event = iota
+	StepEvent
+	StopEvent
+)
diff --git a/internal/plugins/debug.go b/internal/plugins/debug.go
new file mode 100644
index 0000000..77bd4c6
--- /dev/null
+++ b/internal/plugins/debug.go
@@ -0,0 +1,25 @@
+package plugins
+
+import (
+	"log/slog"
+
+	"git.maximhutz.com/max/lambda/internal/engine"
+	"git.maximhutz.com/max/lambda/pkg/lambda"
+)
+
+type Logs struct {
+	logger  *slog.Logger
+	process *engine.Engine
+}
+
+func NewLogs(logger *slog.Logger, process *engine.Engine) *Logs {
+	plugin := &Logs{logger, process}
+	process.On(engine.StopEvent, plugin.Step)
+
+	return plugin
+}
+
+func (t *Logs) Step() {
+	stringified := lambda.Stringify(*t.process.Expression)
+	t.logger.Info("reduction", "tree", stringified)
+}
diff --git a/internal/explanation/tracker.go b/internal/plugins/explanation.go
similarity index 62%
rename from internal/explanation/tracker.go
rename to internal/plugins/explanation.go
index 4650bab..141e8fc 100644
--- a/internal/explanation/tracker.go
+++ b/internal/plugins/explanation.go
@@ -1,6 +1,6 @@
 // Package "explanation" provides a observer to gather the reasoning during the
 // reduction, and present a thorough explanation to the user for each step.
-package explanation
+package plugins
 
 import (
 	"fmt"
@@ -10,23 +10,23 @@ import (
 )
 
 // Track the reductions made by a reduction proess.
-type Tracker struct {
+type Explanation struct {
 	process *engine.Engine
 }
 
 // Attaches a new explanation tracker to a process.
-func Track(process *engine.Engine) *Tracker {
-	tracker := &Tracker{process: process}
-	process.On("start", tracker.Start)
-	process.On("step", tracker.Step)
+func NewExplanation(process *engine.Engine) *Explanation {
+	plugin := &Explanation{process: process}
+	process.On(engine.StartEvent, plugin.Start)
+	process.On(engine.StepEvent, plugin.Step)
 
-	return tracker
+	return plugin
 }
 
-func (t *Tracker) Start() {
+func (t *Explanation) Start() {
 	fmt.Println(lambda.Stringify(*t.process.Expression))
 }
 
-func (t *Tracker) Step() {
+func (t *Explanation) Step() {
 	fmt.Println("    =", lambda.Stringify(*t.process.Expression))
 }
diff --git a/internal/performance/tracker.go b/internal/plugins/performance.go
similarity index 68%
rename from internal/performance/tracker.go
rename to internal/plugins/performance.go
index a4d21ee..0dc69d4 100644
--- a/internal/performance/tracker.go
+++ b/internal/plugins/performance.go
@@ -1,28 +1,34 @@
 // Package "performance" provides a tracker to observer CPU performance during
 // execution.
-package performance
+package plugins
 
 import (
 	"os"
 	"path/filepath"
 	"runtime/pprof"
+
+	"git.maximhutz.com/max/lambda/internal/engine"
 )
 
 // Observes a reduction process, and publishes a CPU performance profile on
 // completion.
-type Tracker struct {
+type Performance struct {
 	File        string
 	filePointer *os.File
 	Error       error
 }
 
 // Create a performance tracker that outputs a profile to "file".
-func Track(file string) *Tracker {
-	return &Tracker{File: file}
+func NewPerformance(file string, process *engine.Engine) *Performance {
+	plugin := &Performance{File: file}
+	process.On(engine.StartEvent, plugin.Start)
+	process.On(engine.StopEvent, plugin.Stop)
+
+	return plugin
 }
 
 // Begin profiling.
-func (t *Tracker) Start() {
+func (t *Performance) Start() {
 	var absPath string
 
 	absPath, t.Error = filepath.Abs(t.File)
@@ -47,7 +53,7 @@ func (t *Tracker) Start() {
 }
 
 // Stop profiling.
-func (t *Tracker) End() {
+func (t *Performance) Stop() {
 	pprof.StopCPUProfile()
 	t.filePointer.Close()
 }
diff --git a/internal/plugins/statistics.go b/internal/plugins/statistics.go
new file mode 100644
index 0000000..00bf5c4
--- /dev/null
+++ b/internal/plugins/statistics.go
@@ -0,0 +1,44 @@
+package plugins
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"git.maximhutz.com/max/lambda/internal/engine"
+	"git.maximhutz.com/max/lambda/internal/statistics"
+)
+
+// An observer, to track reduction performance.
+type Statistics struct {
+	start time.Time
+	steps uint64
+}
+
+// Create a new reduction performance Statistics.
+func NewStatistics(process *engine.Engine) *Statistics {
+	plugin := &Statistics{}
+	process.On(engine.StartEvent, plugin.Start)
+	process.On(engine.StepEvent, plugin.Step)
+	process.On(engine.StopEvent, plugin.Step)
+
+	return plugin
+}
+
+func (t *Statistics) Start() {
+	t.start = time.Now()
+	t.steps = 0
+}
+
+func (t *Statistics) Step() {
+	t.steps++
+}
+
+func (t *Statistics) Stop() {
+	results := statistics.Results{
+		StepsTaken:  t.steps,
+		TimeElapsed: uint64(time.Since(t.start).Milliseconds()),
+	}
+
+	fmt.Fprint(os.Stderr, results.String())
+}
diff --git a/internal/statistics/tracker.go b/internal/statistics/tracker.go
deleted file mode 100644
index 004b1f4..0000000
--- a/internal/statistics/tracker.go
+++ /dev/null
@@ -1,36 +0,0 @@
-package statistics
-
-import (
-	"fmt"
-	"os"
-	"time"
-)
-
-// An observer, to track reduction performance.
-type Tracker struct {
-	start time.Time
-	steps uint64
-}
-
-// Create a new reduction performance tracker.
-func Track() *Tracker {
-	return &Tracker{}
-}
-
-func (t *Tracker) Start() {
-	t.start = time.Now()
-	t.steps = 0
-}
-
-func (t *Tracker) Step() {
-	t.steps++
-}
-
-func (t *Tracker) End() {
-	results := Results{
-		StepsTaken:  t.steps,
-		TimeElapsed: uint64(time.Since(t.start).Milliseconds()),
-	}
-
-	fmt.Fprint(os.Stderr, results.String())
-}
diff --git a/pkg/emitter/emitter.go b/pkg/emitter/emitter.go
index 217da42..594a18a 100644
--- a/pkg/emitter/emitter.go
+++ b/pkg/emitter/emitter.go
@@ -2,53 +2,38 @@ package emitter
 
 import "git.maximhutz.com/max/lambda/pkg/set"
 
-type Observer struct {
-	fn      func()
-	message string
-	emitter *Emitter
+type Emitter[E comparable] interface {
+	On(string, func()) Listener[E]
+	Off(Listener[E])
+	Emit(E)
 }
 
-type Emitter struct {
-	listeners map[string]*set.Set[*Observer]
+type BaseEmitter[E comparable] struct {
+	listeners map[E]*set.Set[Listener[E]]
 }
 
-func Ignore[T any](fn func()) func(T) {
-	return func(T) { fn() }
+func (e *BaseEmitter[E]) On(kind E, fn func()) Listener[E] {
+	if e.listeners[kind] == nil {
+		e.listeners[kind] = set.New[Listener[E]]()
+	}
+
+	listener := &BaseListener[E]{kind, fn}
+	e.listeners[kind].Add(listener)
+	return listener
 }
 
-func (e *Emitter) On(message string, fn func()) *Observer {
-	observer := &Observer{
-		fn:      fn,
-		message: message,
-		emitter: e,
+func (e *BaseEmitter[E]) Emit(event E) {
+	if e.listeners[event] == nil {
+		e.listeners[event] = set.New[Listener[E]]()
 	}
 
-	if e.listeners == nil {
-		e.listeners = map[string]*set.Set[*Observer]{}
-	}
-
-	if e.listeners[message] == nil {
-		e.listeners[message] = set.New[*Observer]()
-	}
-
-	e.listeners[message].Add(observer)
-	return observer
-}
-
-func (o *Observer) Off() {
-	if o.emitter.listeners[o.message] == nil {
-		return
-	}
-
-	o.emitter.listeners[o.message].Remove(o)
-}
-
-func (e *Emitter) Emit(message string) {
-	if e.listeners[message] == nil {
-		return
-	}
-
-	for listener := range *e.listeners[message] {
-		listener.fn()
+	for listener := range e.listeners[event].Items() {
+		listener.Run()
+	}
+}
+
+func New[E comparable]() *BaseEmitter[E] {
+	return &BaseEmitter[E]{
+		listeners: map[E]*set.Set[Listener[E]]{},
 	}
 }
diff --git a/pkg/emitter/listener.go b/pkg/emitter/listener.go
new file mode 100644
index 0000000..43c95c6
--- /dev/null
+++ b/pkg/emitter/listener.go
@@ -0,0 +1,19 @@
+package emitter
+
+type Listener[E comparable] interface {
+	Kind() E
+	Run()
+}
+
+type BaseListener[E comparable] struct {
+	kind E
+	fn   func()
+}
+
+func (l BaseListener[E]) Kind() E {
+	return l.kind
+}
+
+func (l BaseListener[E]) Run() {
+	l.fn()
+}
diff --git a/pkg/set/set.go b/pkg/set/set.go
index 27435d0..f5a70f0 100644
--- a/pkg/set/set.go
+++ b/pkg/set/set.go
@@ -1,5 +1,7 @@
 package set
 
+import "iter"
+
 type Set[T comparable] map[T]bool
 
 func (s *Set[T]) Add(items ...T) {
@@ -34,6 +36,16 @@ func (s Set[T]) ToList() []T {
 	return list
 }
 
+func (s Set[T]) Items() iter.Seq[T] {
+	return func(yield func(T) bool) {
+		for item := range s {
+			if !yield(item) {
+				return
+			}
+		}
+	}
+}
+
 func New[T comparable](items ...T) *Set[T] {
 	result := &Set[T]{}