How do I restrict the type of a text node in a mixed, complex-type element?

[Chishun Kwong]

Hi,

I would like to use xsd to restrict the text in a mixed, complex-type
element. My reading so far seems to say that it is not achievable, is
this true? I find it hard to imagine.

For example, in

<foo>
some text
<bar>some more text</bar>
</foo>

I know I must define foo as an element with a complex type that is
mixed. I also know that restricting the text of <bar/> is relatively
easy, but I cannot find any reference that tells me how to restrict the
text of <foo/>, in this case the current value is "some text".

Thanks in advance,
Chishun Kwong