diff --git a/compiler/Lexer.ts b/compiler/Lexer.ts
index 4067057..fd0f74d 100644
--- a/compiler/Lexer.ts
+++ b/compiler/Lexer.ts
@@ -51,6 +51,19 @@ export class Lexer {
             }
             return { ...this.token("int", pos), intValue: parseInt(textValue) };
         }
+
+        if (this.test("0")) {
+            this.step()
+            if (!this.done() && this.test(/[0-9]/)) {
+                console.error(
+                    `Lexer: invalid number`
+                        + ` at ${pos.line}:${pos.col}`,
+                );
+                return this.token("error", pos);
+            }
+            return { ...this.token("int", pos), intValue: 0};
+        }
+
         if (this.test("\"")) {
             this.step();
             let value = "";
diff --git a/compiler/ast.ts b/compiler/ast.ts
index f6b9d89..30d46bf 100644
--- a/compiler/ast.ts
+++ b/compiler/ast.ts
@@ -46,4 +46,13 @@ export type ExprKind =
     | { type: "null"}
     | { type: "loop", body: Expr }
     | { type: "block", stmts: Stmt[], expr?: Expr }
-    ;
\ No newline at end of file
+    ;
+
+export type Sym = {
+    ident: string,
+    type: "let" | "fn" | "fn_param" | "builtin",
+    pos?: Pos,
+    stmt?: Stmt,
+    param?: Param,
+}
+