Fix setting negative index values

guidance/_variable_stack.py

@@ -82,7 +82,7 @@ def __setitem__(self, key, value):
             found = True
             for part in parts:
                 if part.endswith("]"):
-                    if re.match(r"['\"0-9].*", part):
+                    if re.match(r"[-'\"0-9].*", part):
                         var_part = ast.literal_eval(part[:-1])
                     else:
                         var_part = self.get(part[:-1])