Schema Question

[Dan White <ygor@comcast.net>]

I need a pointer or two about creating a schema.

I have a situation where the incoming XML will have an element/attribute
specifying one of four enumerated choices.  Depending on the value of that
choice, the elements that follow will be different.

Like this:
<foo>
  <mode>one</mode>
  <data1>aaa</data1>
  <data2>bbb</data2>
</foo

<foo>
  <mode>two</mode>
  <data3>ccc</data3>
  <data4>ddd</data4>
</foo>

or
<foo mode="one">
  <data1>aaa</data1>
  <data2>bbb</data2>
</foo

<foo mode="two">
  <data3>ccc</data3>
  <data4>ddd</data4>
</foo>

How does one set up the schema to validate the appropriate sequence of
elements based on the value of the "controlling" element/attribute ?

Thanks in advance.