Fix bug in error handling where __sub would be clobbered.
diff --git a/src/lisp/lisp.h b/src/lisp/lisp.h
index 430e526..eff35f9 100644
--- a/src/lisp/lisp.h
+++ b/src/lisp/lisp.h
@@ -147,6 +147,8 @@
  */
 void pop_pool(unsigned char pool);
 
+void add_to_pool(value_t form);
+
 /**
  * @returns true if pool is still alive (in scope).
  */