Specifying an empty string as a simpleType restriction option

Hello,

I think I'm making a mistake below--XMLSpy isn't accepting this simpleType.

For the following simpleType, I'm trying to say that widgetType may
have any of the three possible values:

1.) abc
2.) bcd
3.) ( an empty string )

<xs:simpleType name="widgetType">
    <xs:restriction base="xs:string">
       <xs:pattern value="abc|bcd|" />
    </xs:restriction>
</xs:simpleType>

Apparently, though, the second bar to the ending quote is not an
acceptable way to specify the empty string.  Does anyone know what I'm
missing here--how I can specify an empty string as one of the three
acceptable options?

Thanks,
Glen