> Where the "()" select will evaluate to an empty sequence when none was
> supplied.  I understand that it could be any valid xpath expression.

I forget whether you are using xslt2 or 1, if using one you can't use ()
but you can use /.. to swlwct an empty node set.

> And the param "my_param" is not supplied you will end up with a copy
> of the document source "/" or current context "." (respectively).  Do
> I have this correct?

for a top level param the current context is the root node so . and /
are the same thing. You don't get a copy the vaiable is bound to the
same node. so for example the counting the node set 
count($my_paran | / )
of the parameter and the document root would return 1 not 2. In
particular defining a parameter this way is not expensive (whereas doing
<xsl:copy-of select="/"/> is expensive and generates a copy of the input
(if it is being copied into a variable.

David

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