怎么用XSD写下面的枚举常量。重点是进行赋值。

typedef enum _SERVER_STATE {
SERVER_ERROR = -1,
SERVER_MAINTENANCE = 0,
SERVER_OK = 1
}SERVER_STATE;

<?xml version="1.0" encoding="UTF-8"?>


xs:complexType
xs:sequence



/xs:sequence
/xs:complexType
/xs:element



/xs:schema

 <?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">
  <xs:element name="_SERVER_STATE">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="SERVER_ERROR"/>
        <xs:element ref="SERVER_MAINTENANCE"/>
        <xs:element ref="SERVER_OK"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:element name="SERVER_ERROR" type="xs:integer"/>
  <xs:element name="SERVER_MAINTENANCE" type="xs:integer"/>
  <xs:element name="SERVER_OK" type="xs:integer"/>
</xs:schema>