This will bring down the whole shell, since we're in a builtin. Better to return
an error instead, since this is unspecified:
> If an application sets or unsets the value of PWD, the behavior of pwd is
This is an assertion because it'd be a programming error in mrsh, for
> Note that this is mostly a placation, because mrsh manages the PWD
> variable and ensures that none of the conditions -P or -L are meant to
> address will occur.