diff --git a/packages/compiler/scripts/modparse.ts b/packages/compiler/scripts/modparse.ts
index 1317e31e..6b2a70e0 100644
--- a/packages/compiler/scripts/modparse.ts
+++ b/packages/compiler/scripts/modparse.ts
@@ -13,9 +13,9 @@ const WASM_NUMTYPES = [0x7c, 0x7d, 0x7e, 0x7f];
 const WASM_VECTYPE = 0x7b;
 const WASM_REFTYPES = [0x6f, 0x70];
 
-function checkNotNull<T>(x): NonNullable<T> {
+function checkNotNull<T>(x: T): NonNullable<T> {
   assert(x !== null, 'unexpected null value');
-  return x;
+  return x as NonNullable<T>;
 }
 
 function skipPreamble(bytes: Uint8Array): void {
diff --git a/packages/compiler/tsconfig.json b/packages/compiler/tsconfig.json
index ca741ee4..8717b89f 100644
--- a/packages/compiler/tsconfig.json
+++ b/packages/compiler/tsconfig.json
@@ -18,8 +18,12 @@
   },
   "include": [
     "src/**/*",
+    "test/**/*.ts",
+    "scripts/**/*.ts",
     "build/ohmRuntime.wasm_sections.ts",
     "build/ohm-grammar-wasm.ts",
-    "index.ts"
-  ]
+    "index.ts",
+    "tsdown.config.ts"
+  ],
+  "exclude": ["test/_stage-1-cst.spec.ts"]
 }
diff --git a/packages/ohm-js/index.d.ts b/packages/ohm-js/index.d.ts
index 1fbeb582..e41cdc00 100644
--- a/packages/ohm-js/index.d.ts
+++ b/packages/ohm-js/index.d.ts
@@ -105,9 +105,16 @@ export class PExpr {
   toString(): string;
   toDisplayString(): string;
 }
-export class Terminal extends PExpr {}
-export class Range extends PExpr {}
-export class Param extends PExpr {}
+export class Terminal extends PExpr {
+  obj: string;
+}
+export class Range extends PExpr {
+  from: string;
+  to: string;
+}
+export class Param extends PExpr {
+  index: number;
+}
 export class Alt extends PExpr {
   terms: PExpr[];
 }
@@ -116,14 +123,25 @@ export class Splice extends Alt {}
 export class Seq extends PExpr {
   factors: PExpr[];
 }
-export class Iter extends PExpr {}
+export class Iter extends PExpr {
+  expr: PExpr;
+}
 export class Star extends Iter {}
 export class Plus extends Iter {}
 export class Opt extends Iter {}
-export class Not extends PExpr {}
-export class Lookahead extends PExpr {}
-export class Lex extends PExpr {}
-export class Apply extends PExpr {}
+export class Not extends PExpr {
+  expr: PExpr;
+}
+export class Lookahead extends PExpr {
+  expr: PExpr;
+}
+export class Lex extends PExpr {
+  expr: PExpr;
+}
+export class Apply extends PExpr {
+  ruleName: string;
+  args: PExpr[];
+}
 export class UnicodeChar extends PExpr {}
 export class CaseInsensitiveTerminal extends PExpr {}
 
diff --git a/packages/ohm-js/scripts/data/index.d.ts.template b/packages/ohm-js/scripts/data/index.d.ts.template
index 94ddb9bd..b23a251d 100644
--- a/packages/ohm-js/scripts/data/index.d.ts.template
+++ b/packages/ohm-js/scripts/data/index.d.ts.template
@@ -104,9 +104,16 @@ export class PExpr {
   toString(): string;
   toDisplayString(): string;
 }
-export class Terminal extends PExpr {}
-export class Range extends PExpr {}
-export class Param extends PExpr {}
+export class Terminal extends PExpr {
+  obj: string;
+}
+export class Range extends PExpr {
+  from: string;
+  to: string;
+}
+export class Param extends PExpr {
+  index: number;
+}
 export class Alt extends PExpr {
   terms: PExpr[];
 }
@@ -115,14 +122,25 @@ export class Splice extends Alt {}
 export class Seq extends PExpr {
   factors: PExpr[];
 }
-export class Iter extends PExpr {}
+export class Iter extends PExpr {
+  expr: PExpr;
+}
 export class Star extends Iter {}
 export class Plus extends Iter {}
 export class Opt extends Iter {}
-export class Not extends PExpr {}
-export class Lookahead extends PExpr {}
-export class Lex extends PExpr {}
-export class Apply extends PExpr {}
+export class Not extends PExpr {
+  expr: PExpr;
+}
+export class Lookahead extends PExpr {
+  expr: PExpr;
+}
+export class Lex extends PExpr {
+  expr: PExpr;
+}
+export class Apply extends PExpr {
+  ruleName: string;
+  args: PExpr[];
+}
 export class UnicodeChar extends PExpr {}
 export class CaseInsensitiveTerminal extends PExpr {}