Fixed!

src/TraitExpressionBuilder.ts

@@ -9,7 +9,7 @@ class TraitExpressionBuilder<
     OwnTraits extends Trait<any, any, any>[],
     AllTraits extends Trait<any, any, any>[],
 > {
-    constructor(public expression: TraitExpression<Super, OwnTraits, AllTraits>) {}
+    constructor(private expression: TraitExpression<Super, OwnTraits, AllTraits>) {}
 
     extends<
         NewSuper extends AbstractClass<any>
@@ -30,7 +30,7 @@ class TraitExpressionBuilder<
     ): TraitExpressionBuilder<
         Super,
         [...OwnTraits, ...Traits],
-        [...AllTraits, SpreadSupertraits<Traits>]
+        [...AllTraits, ...SpreadSupertraits<Traits>]
     > {
         return new TraitExpressionBuilder(
             new TraitExpression(

src/tests.ts

@@ -1,4 +1,4 @@
-import { Implements, TraitExpression, abstract, expression, trait } from "."
+import { Implements, abstract, expression, trait } from "."
 
 
 const PrintsHelloOnNew = trait(
@@ -57,8 +57,6 @@ const builder = expression
         StatefulSubscription,
     )
 
-type T = TraitExpression.AllTraits<typeof builder.expression>
-
 const exp = builder.get()
 type Abs = Implements<typeof exp>