From 8e0472a5ffc5319a0a2a7cd1fc5c3514b98a50a6 Mon Sep 17 00:00:00 2001
From: Simon Forman <sforman@hushmail.com>
Date: Fri, 9 Apr 2021 17:49:44 -0700
Subject: [PATCH] Correct swap.

---
 joy/utils/generated_library.py | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/joy/utils/generated_library.py b/joy/utils/generated_library.py
index 50ce8c2..071cce4 100644
--- a/joy/utils/generated_library.py
+++ b/joy/utils/generated_library.py
@@ -340,7 +340,10 @@ def swap(stack):
     (a1 a2 -- a2 a1)
 
   """
-  (a2, (a1, s23)) = stack
+  try:
+    (a2, (a1, s23)) = stack
+  except ValueError:
+    raise StackUnderflowError('Not enough values on stack.')
   return (a1, (a2, s23))