XSD: require attributes to have specific values

I'm trying to make an XSD that specifies that an <a> element must have 4 child <b> elements, which contain the c attributes 1 through 4, as follows:

Valid:

<a>
  <b c="1" d="valueof1" />
  <b c="2" d="valueof2" />
  <b c="3" d="valueof3" />
  <b c="4" d="valueof4" />
</a>

Not valid:

&开发者_C百科lt;a>
  <b c="1" d="valueof1" />
  <b c="1" d="valueof1_other" />
  <b c="3" d="valueof3" />
  <b c="4" d="valueof4" />
</a>

Not valid:

<a>
  <b c="1" d="valueof1" />
  <b c="2" d="valueof2" />
  <b c="3" d="valueof3" />
  <b c="4" d="valueof4" />
  <b c="5" d="valueof5" />
</a>

Is this possible? The closest thing I could find is the all element, but that only appears to work for specifying unique elements, not attribute values.

---

You can enforce the first rule by declaring element b with minOccurs=4 and maxOccurs=4, like so:

<xs:element name="a">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="b" minOccurs="4" maxOccurs="4"/>
    </xs:sequence>
  </xs:complexType>
</xs:element>

However, you can not use XSD to enforce the second rule. If you really want to do that, you can, for example, replace the 4 b elements with elements b1, b2, b3, and b4, each of which would implicitly represent the c attributes 1 through 4, respectively.