[plt-scheme] <select multiple> ... </select> as an xexpr

How would I write something like this using xexpr?  The problem being
that MULTIPLE doesn't have an ="..." attatched to it.

<SELECT NAME="toppings" MULTIPLE SIZE=5>
<OPTION VALUE="mushrooms">mushrooms
<OPTION VALUE="greenpeppers">green peppers
<OPTION VALUE="onions">onions
<OPTION VALUE="tomatoes">tomatoes
<OPTION VALUE="olives">olives
</SELECT>

-Andrey