Control one attribute depending on another?

[Thomas]

Dear all,

I am new to XML Schema. I would like to know if the following syntax
control of an XML file can be achieved by XML Schema:

Given soemthing like

<my_element first_att="A" second_att="X"/>
<my_element first_att="B" second_att="Y"/>

can I require that if the first attribute has the value "A", the second
may only have the vaue "X", and if the first attribute has the value "B"
the second may only have the value "Y".

Of course, what I actually want to do is a bit more complex. I guess the
basic question is: can I control the content of one attribute in
dependance from the content of another attribute?

Thanks for your help,

Thomas