Correspondant to HTML "Input box" in XSL

Concerning the following problem I couldn find any suiting examples to adapt
to my use:

XML instance:

<list>
 <item att="a" />
 <item att="b" />
 <item att="a" />
 <item att="z" />
 <item att="z" />
</list>

The XSL  is designed in order to show me all items depending on the value of
"att", e.g. all elements with att="a".

Question: How can I pass the parameter "a" (or "b" or "z" or "pippo") to the
XSL in order to have it select the appropriate items?

Matthias


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