Processing an element only if there are no better options

Sorry for the vague title, but I don't know how I could word the problem is
a couple of words.

Imagine a couple of dataset like these:

[doc #1]
<ProductGroup>
        <Product Type="Type1">[...]</Product>
        <Product Type="Type2">[...]</Product>
        <Product Type="Type3">[...]</Product>
</ProductGroup>
[/doc #1]


[doc #2]
<ProductGroup>
        <Product Type="Type2">[...]</Product>
</ProductGroup>
[/doc #2]


What I need to do is only extract the contents of _1_ Product in an order of
preference. 

i.e. If there is a Type1 product, then don't output Type2 or Type3.
However, if there is no Type1, then output Type2, but not Type3 if it's
present.


Any help would be greatly appreciated.

Thanks


        Dave

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