Less v2 does not compile Twitter’s Bootstrap 2.x

I was able to avoid the error without modifying Bootstrap files by creating a new mixin that loaded after the Bootstrap mixins:

#grid {
    .core  {
        .span(@gridColumns) {
            width: (@gridColumnWidth * @gridColumns) + (@gridGutterWidth * (@gridColumns - 1));
        }
    }
};

This was better for us as we avoid patching contrib packages.