I have some XSD that looks something like this:

```
<element name="a">
  <complexType>
    <sequence>
      <element name="b" type="t:typ" minOccurs="1" maxOccurs="unbounded" />
      <element name="c" type="t:typ" minOccurs="1" maxOccurs="unbounded" />
    </sequence>
  </complexType>
</element>

```

How would I change it so that instead of a sequence, I could allow the tags b and c to be jumbled in any order, e.g. how would I make this valid?..

```
<a>
  <b />
  <c />
  <b />
  <c />
  <b />
  <b />
</a>

```

The 'all' option sounded promising, but it seems to only allow no more than one of each of the child elements.