Text centered in box of fixed height & width

[Tod Olson]

Given a box of fixed height and width, how does one center text in it?

Horizontal centering is easy:

    @Box 1cm @High 5cm @Wide @Center text

I've not figured out how to vertically center.  I've tried @VShift at
various places, but the text always lands at the top, like so:

        +--------+
        |  text  |
        |        |
        +--------+

Hints or ideas appreciated.  Thanks.

Tod A. Olson                        "How do you know I'm mad?" said Alice.
address@hidden               "If you weren't mad, you wouldn't have
The University of Chicago Library    come here," said the Cat.