diff --git a/automod/engine.go b/automod/engine.go
index c1bd0919..23641e65 100644
--- a/automod/engine.go
+++ b/automod/engine.go
@@ -5,25 +5,24 @@ import (
 	"fmt"
 	"log/slog"
 	"strings"
-	"sync"
 
 	appbsky "github.com/bluesky-social/indigo/api/bsky"
 	"github.com/bluesky-social/indigo/atproto/identity"
 	"github.com/bluesky-social/indigo/atproto/syntax"
-	"github.com/bluesky-social/indigo/xrpc"
 )
 
 // runtime for executing rules, managing state, and recording moderation actions.
 //
 // TODO: careful when initializing: several fields should not be null or zero, even though they are pointer type.
 type Engine struct {
-	Logger  *slog.Logger
+	Logger    *slog.Logger
 	Directory identity.Directory
 	// current rule sets. will eventually be possible to swap these out at runtime
-	RulesMap  sync.Map
+	// TODO: RulesMap  sync.Map
 	// used to persist moderation actions in mod service (optional)
-	AdminClient *xrpc.Client
-	CountStore  CountStore
+	// TODO: AdminClient *xrpc.Client
+	Counters CountStore
+	Sets     SetStore
 }
 
 func (e *Engine) ProcessIdentityEvent(ctx context.Context, t string, did syntax.DID) error {
@@ -92,7 +91,7 @@ func (e *Engine) ProcessRecord(ctx context.Context, did syntax.DID, path string,
 
 func (e *Engine) NewPostEvent(ident *identity.Identity, path string, post *appbsky.FeedPost) PostEvent {
 	return PostEvent{
-		RecordEvent {
+		RecordEvent{
 			Event{
 				Engine:  e,
 				Account: AccountMeta{Identity: ident},
@@ -119,10 +118,10 @@ func (e *Engine) NewRecordEvent(ident *identity.Identity, path string, rec any)
 }
 
 func (e *Engine) GetCount(key, period string) (int, error) {
-	return e.CountStore.GetCount(context.TODO(), key, period)
+	return e.Counters.GetCount(context.TODO(), key, period)
 }
 
-func (e *Engine) InSet(setName, val string) (bool, error) {
-	// XXX: implement
-	return false, nil
+// checks if `val` is an element of set `name`
+func (e *Engine) InSet(name, val string) (bool, error) {
+	return e.Sets.InSet(context.TODO(), name, val)
 }
diff --git a/automod/setstore.go b/automod/setstore.go
new file mode 100644
index 00000000..d113d1bb
--- /dev/null
+++ b/automod/setstore.go
@@ -0,0 +1,30 @@
+package automod
+
+import (
+	"context"
+	"fmt"
+)
+
+type SetStore interface {
+	InSet(ctx context.Context, name, val string) (bool, error)
+}
+
+// TODO: this implementation isn't race-safe (yet)!
+type MemSetStore struct {
+	Sets map[string]map[string]bool
+}
+
+func NewMemSetStore() MemSetStore {
+	return MemSetStore{
+		Sets: make(map[string]map[string]bool),
+	}
+}
+
+func (s MemSetStore) InSet(ctx context.Context, name, val string) (bool, error) {
+	set, ok := s.Sets[name]
+	if !ok {
+		return false, fmt.Errorf("not a known set: %s", name)
+	}
+	_, ok = set[val]
+	return ok, nil
+}
diff --git a/cmd/hepa/otel.go b/cmd/hepa/otel.go
index 918bd1b7..b551325c 100644
--- a/cmd/hepa/otel.go
+++ b/cmd/hepa/otel.go
@@ -1,10 +1,10 @@
 package main
 
 import (
-	"os"
-	"log/slog"
-	"log"
 	"context"
+	"log"
+	"log/slog"
+	"os"
 	"time"
 
 	"go.opentelemetry.io/otel"
diff --git a/cmd/hepa/server.go b/cmd/hepa/server.go
index 682f84f9..332f3e61 100644
--- a/cmd/hepa/server.go
+++ b/cmd/hepa/server.go
@@ -38,9 +38,10 @@ func NewServer(dir identity.Directory, config Config) (*Server, error) {
 	}
 
 	engine := automod.Engine{
-		Logger: logger,
+		Logger:    logger,
 		Directory: dir,
-		CountStore: automod.NewMemCountStore(),
+		Counters:  automod.NewMemCountStore(),
+		Sets:      automod.NewMemSetStore(),
 		// TODO: RulesMap (loaded/config from somewhere)
 		// TODO: AdminClient (XRPC with mod access)
 	}