diff --git a/README.md b/README.md
index 711da17..112f5f4 100644
--- a/README.md
+++ b/README.md
@@ -137,8 +137,9 @@ Generated documentation for the latest version of the Go implementation can be a
 If you're looking to integrate Cedar into a production system, please be sure the read the [security best practices](https://docs.cedarpolicy.com/other/security.html)
 
 ## Backward Compatibility Considerations
-
-x/exp - code in this directory is not subject to the semantic version constraints of the rest of the module and breaking changes may be made at any time
+- `x/exp` - code in this directory is not subject to the semantic versioning constraints of the rest of the module and breaking changes may be made at any time.
+- Variadics may be added to functions that do not have them to expand the arguments of a function or method.
+- Concrete types may be replaced with compatible interfaces to expand the variety of arguments a function or method can take.
 
 ## Change log
 
diff --git a/authorize.go b/authorize.go
index 302d6d7..d8164fa 100644
--- a/authorize.go
+++ b/authorize.go
@@ -18,7 +18,7 @@ const (
 
 // IsAuthorized uses the combination of the PolicySet and Entities to determine
 // if the given Request to determine Decision and Diagnostic.
-func (p PolicySet) IsAuthorized(entities types.EntityMap, req Request) (Decision, Diagnostic) {
+func (p PolicySet) IsAuthorized(entities types.EntityGetter, req Request) (Decision, Diagnostic) {
 	env := eval.Env{
 		Entities:  entities,
 		Principal: req.Principal,
diff --git a/internal/eval/evalers.go b/internal/eval/evalers.go
index 4ca1bce..d8cded9 100644
--- a/internal/eval/evalers.go
+++ b/internal/eval/evalers.go
@@ -22,7 +22,7 @@ func zeroValue() types.Value {
 }
 
 type Env struct {
-	Entities                    types.EntityMap
+	Entities                    types.EntityGetter
 	Principal, Action, Resource types.Value
 	Context                     types.Value
 }
@@ -754,7 +754,7 @@ func (n *attributeAccessEval) Eval(env Env) (types.Value, error) {
 		if vv == unspecified {
 			return zeroValue(), fmt.Errorf("cannot access attribute `%s` of %w", n.attribute, errUnspecifiedEntity)
 		}
-		rec, ok := env.Entities[vv]
+		rec, ok := env.Entities.Get(vv)
 		if !ok {
 			return zeroValue(), fmt.Errorf("entity `%v` %w", vv.String(), errEntityNotExist)
 		}
@@ -792,7 +792,7 @@ func (n *hasEval) Eval(env Env) (types.Value, error) {
 	var record types.Record
 	switch vv := v.(type) {
 	case types.EntityUID:
-		if rec, ok := env.Entities[vv]; ok {
+		if rec, ok := env.Entities.Get(vv); ok {
 			record = rec.Attributes
 		}
 	case types.Record:
@@ -861,12 +861,12 @@ func entityInOne(env Env, entity types.EntityUID, parent types.EntityUID) bool {
 	var todo []types.EntityUID
 	var candidate = entity
 	for {
-		if fe, ok := env.Entities[candidate]; ok {
+		if fe, ok := env.Entities.Get(candidate); ok {
 			if fe.Parents.Contains(parent) {
 				return true
 			}
 			fe.Parents.Iterate(func(k types.EntityUID) bool {
-				p, ok := env.Entities[k]
+				p, ok := env.Entities.Get(k)
 				if !ok || p.Parents.Len() == 0 || k == entity || known.Contains(k) {
 					return true
 				}
@@ -890,12 +890,12 @@ func entityInSet(env Env, entity types.EntityUID, parents mapset.Container[types
 	var todo []types.EntityUID
 	var candidate = entity
 	for {
-		if fe, ok := env.Entities[candidate]; ok {
+		if fe, ok := env.Entities.Get(candidate); ok {
 			if fe.Parents.Intersects(parents) {
 				return true
 			}
 			fe.Parents.Iterate(func(k types.EntityUID) bool {
-				p, ok := env.Entities[k]
+				p, ok := env.Entities.Get(k)
 				if !ok || p.Parents.Len() == 0 || k == entity || known.Contains(k) {
 					return true
 				}
diff --git a/internal/eval/partial.go b/internal/eval/partial.go
index 07106cd..33dede4 100644
--- a/internal/eval/partial.go
+++ b/internal/eval/partial.go
@@ -504,7 +504,7 @@ func (n *partialHasEval) Eval(env Env) (types.Value, error) {
 	var record types.Record
 	switch vv := v.(type) {
 	case types.EntityUID:
-		if rec, ok := env.Entities[vv]; ok {
+		if rec, ok := env.Entities.Get(vv); ok {
 			record = rec.Attributes
 		}
 	case types.Record:
diff --git a/types/entity_map.go b/types/entity_map.go
index 70b9034..7de8039 100644
--- a/types/entity_map.go
+++ b/types/entity_map.go
@@ -8,6 +8,13 @@ import (
 	"golang.org/x/exp/maps"
 )
 
+// An EntityGetter is an interface for retrieving an Entity by EntityUID.
+type EntityGetter interface {
+	Get(uid EntityUID) (Entity, bool)
+}
+
+var _ EntityGetter = EntityMap{}
+
 // An EntityMap is a collection of all the entities that are needed to evaluate
 // authorization requests.  The key is an EntityUID which uniquely identifies
 // the Entity (it must be the same as the UID within the Entity itself.)
@@ -37,3 +44,8 @@ func (e *EntityMap) UnmarshalJSON(b []byte) error {
 func (e EntityMap) Clone() EntityMap {
 	return maps.Clone(e)
 }
+
+func (e EntityMap) Get(uid EntityUID) (Entity, bool) {
+	ent, ok := e[uid]
+	return ent, ok
+}
diff --git a/types/entity_map_test.go b/types/entity_map_test.go
index 507e38d..a8a2afe 100644
--- a/types/entity_map_test.go
+++ b/types/entity_map_test.go
@@ -22,6 +22,21 @@ func TestEntities(t *testing.T) {
 		testutil.Equals(t, clone, e)
 	})
 
+	t.Run("Get", func(t *testing.T) {
+		t.Parallel()
+		ent := types.Entity{
+			UID:        types.NewEntityUID("Type", "id"),
+			Attributes: types.NewRecord(types.RecordMap{"key": types.Long(42)}),
+		}
+		e := types.EntityMap{
+			ent.UID: ent,
+		}
+		got, ok := e.Get(ent.UID)
+		testutil.Equals(t, ok, true)
+		testutil.Equals(t, got, ent)
+		_, ok = e.Get(types.NewEntityUID("Type", "id2"))
+		testutil.Equals(t, ok, false)
+	})
 }
 
 func TestEntitiesJSON(t *testing.T) {