From 7957136ac3526e9e38201c226ccd7e2b642784f4 Mon Sep 17 00:00:00 2001
From: Simon Forman <sforman@hushmail.com>
Date: Fri, 9 Apr 2021 17:46:45 -0700
Subject: [PATCH] Correct swaack.

---
 joy/utils/generated_library.py | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/joy/utils/generated_library.py b/joy/utils/generated_library.py
index 8496db6..50ce8c2 100644
--- a/joy/utils/generated_library.py
+++ b/joy/utils/generated_library.py
@@ -324,7 +324,12 @@ def swaack(stack):
     ([...1] -- [...0])
 
   """
-  (s1, s0) = stack
+  try:
+    (s1, s0) = stack
+  except ValueError:
+    raise StackUnderflowError('Not enough values on stack.')
+  if not isinstance(s1, tuple):
+    raise NotAListError('Not a list.')
   return (s0, s1)