XPath (XSLT) match only specific structure

How do I match only a specific structure like:

<sandwich>
    <bacon />
    <lettuce />
    <tomato />
</sandwich>

I want to replace it with something like

<sandwich>
    <cheese />
    <bacon />
    <lettuce />
    <tomato />
    <mayo />
</sandwich>

But not effect any other similar "sandwiches", such as:

<sandwich>
    <bacon />
    <egg />
</sandwich>

More specifically, I have

<view>
    <layout>
        <sidebar>Some content and nodes</sidebar>
        <content>Some more content and nodes</content>
    </layout>
    <layout>
        <content>Some content and nodes</content>
        <sidebar>Some more content and nodes</sidebar>
    </layout>
</view>

And I want to transform it into:

<view>
    <foo>
        <bar>Some content and nodes</bar>
        <baz>Some more content and nodes</baz>
    </foo>
    <layout>
        <content>Some content and nodes</content>
        <sidebar>Some more content and nodes</sidebar>
    </layout>
</view>

(using XSLT)