diff --git a/src/TraitExpressionBuilder.ts b/src/TraitExpressionBuilder.ts
index 2885fe6..3591c74 100644
--- a/src/TraitExpressionBuilder.ts
+++ b/src/TraitExpressionBuilder.ts
@@ -33,7 +33,7 @@ export class TraitExpressionBuilder<
         return traits.flatMap(trait => [
             ...trait.superExpression.traits,
             trait,
-        ]) as TraitExpressionBuilder.SpreadSupertraits<T>
+        ]) as readonly Trait<any, any, any, any>[] as TraitExpressionBuilder.SpreadSupertraits<T>
     }
 
     static traitsUniq<
@@ -49,7 +49,7 @@ export class TraitExpressionBuilder<
     >(
         traits: T
     ) {
-        return uniq(traits) as TraitExpressionBuilder.TraitsUniq<T>
+        return uniq(traits) as readonly Trait<any, any, any, any>[] as TraitExpressionBuilder.TraitsUniq<T>
     }
 
 
@@ -142,30 +142,30 @@ export class TraitExpressionBuilder<
 
 export namespace TraitExpressionBuilder {
     export type SpreadSupertraits<Traits> = (
-        Traits extends [
+        Traits extends readonly [
             infer El extends Trait<any, any, any, any>,
             ...infer Rest,
         ]
-            ? [
+            ? readonly [
                 ...Trait.Supertraits<El>,
                 El,
                 ...SpreadSupertraits<Rest>,
             ]
-            : []
+            : readonly []
     )
 
     export type TraitsUniq<Traits> = (
-        Traits extends [
+        Traits extends readonly [
             ...infer Rest,
             infer El extends Trait<any, any, any, any>,
         ]
             ? IsTraitInTupleFromRight<Rest, El> extends true
                 ? TraitsUniq<Rest>
-                : [...TraitsUniq<Rest>, El]
-            : []
+                : readonly [...TraitsUniq<Rest>, El]
+            : readonly []
     )
     type IsTraitInTupleFromRight<Traits, T> = (
-        Traits extends [...infer Rest, infer El]
+        Traits extends readonly [...infer Rest, infer El]
             ? IsEqual<El, T> extends true
                 ? true
                 : IsTraitInTupleFromRight<Rest, T>