You have to be careful not to pass mutable stringref's into anything that doesn't expect one (which is most things!). Indeed there's no need to use an AutoStringRef to hold the mutable copy here... You can do:
```
/* UNCHECKED */ MCStringMutableCopyAndRelease(t_value, t_value);
/* UNCHECKED */ MCStringRemove(t_value ...);
/* UNCHECKED */ MCStringCopyAndRelease(t_value, t_value);
... -> setstringprop(...)
MCValueRelease(t_value)