gittup/tup — pull request #108 by Rendaw (2013-04-30 15:56:01+00:00)
Title: Fix for += in builtins

It looks like you need to make ENV_ references explicit since it doesn't necessarily appear in upvals and may appear at any index in there. I think singlevaraux provides an example of grabbing ENV.

This reproduces the issue:

```lua
local x = '0'
function y()
x = '1'
local z
z += x
end
```