It would be better here to not use an autostring ref for efficiency reasons (error handling excluded)...
```
MCU_geturl(ctxt, p_chunk.url, t_old_data);
MCStringMutableCopyAndRelease(t_old_data, t_old_data);
if (after)
  MCStringAppend(t_old_data, p_value)
else
  MCStringPrepend(t_old_data, p_value)
```
This (when libfoundation is properly implemented) will mean the least amount of copying.