list of pre-defined value

How can I define a space separated list of values for an attribute using 
NMTOKENS. The values one may use must be predefined.

The below code does not seem to work. The attribute value does not allow for 
the containment of a LIST of the values; yet appears to be restricted to one 
value only.

<xs:simpleType>
<xs:restriction base="xs:NMTOKENS">
<xs:enumeration value="value1"/>
<xs:enumeration value="value2"/>
<xs:enumeration value="value3"/>
<xs:enumeration value="value4"/>
<xs:enumeration value="value5"/>
</xs:restriction>
</xs:simpleType>

many thanks
d.

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