conditional schemas

Hello,

Can anyone help me with the following:
I have two xml documents that are quite similar, except for the presence of
one particular tag. I would like to use the same schema to validate both
these documents. Is there a way to make a schema conditional? Can I in some
way express that for instance if the document contains attribute value "x",
that a certain tag should be present, and if the document contains attribute
value "y", that it cannot contain that same tag?

example:

<actions name="access">
    ...
    ==> the document has to contain tag <tag>
    ...
</actions>

<actions name="settings">
    ...
    ==> the document may not contain tag <tag>
    ...
</actions>

In this case, can I express in the schema that if the value of name is
access, the <tag> should be present, and if the value of name is settings,
it shouldn't?

Thanks a lot,
Lieve