diff --git a/implementations/javascript/packages/core/src/pexpr.ts b/implementations/javascript/packages/core/src/pexpr.ts
index e5be35e..9d15420 100644
--- a/implementations/javascript/packages/core/src/pexpr.ts
+++ b/implementations/javascript/packages/core/src/pexpr.ts
@@ -82,6 +82,10 @@ export class BaseCompound<I> {
         });
     }
 
+    __as_preserve__(): Value {
+        return this.preservesValues();
+    }
+
     map<R>(f: (item: Positioned<I>, index: number) => R, offset = 0): R[] {
         const result: R[] = [];
         for (let i = offset; i < this.exprs.length; i++) {
@@ -106,11 +110,7 @@ export class BaseCompound<I> {
     }
 }
 
-export class Document extends BaseCompound<Expr> {
-    __as_preserve__(): Value {
-        return this.preservesValues();
-    }
-}
+export class Document extends BaseCompound<Expr> {}
 
 export class Annotations extends BaseCompound<SimpleExpr> {
     wrap(v: Value): Value {
@@ -352,4 +352,4 @@ export function asPreserves<T extends Embeddable>(
     }
 
     return walk(p);
-}
\ No newline at end of file
+}