Piping using foo &| bar will pipe both stdout and stderr to bar.
With MULTIOS enabled, multiple outputs to the same file descriptor can be provided, similar to using tee.
In other words:
$ date >foo >bar
will output date to both foo and bar. (Piping it will also work since it implicitly is like redirecting).
cd old new
cd old new will substitute new into old.
I.e. cd foo bar in a working directory like /a/b/foo/c will change to /a/b/bar/c.
vared SOME_ENVIRONMENT_VARIABLE will load the environment variable into a line editor, with the output saved back to the environment variable.
mailpath specifies the set of paths to look at for new mail.