relative URI in href

I can not determine the "base URI of the containing element node" as
descripte in http://www.w3.org/TR/xslt20/#uri-references.

I aim to include a stylesheet from the very same location as the
including one. While developing I'm initiating the transformation from
my IDE. Later on it will be part of a jar file. If I make the
href="/.../to-be-included.xsl" absolut, it works fine. If I omit the
directory path like href="to-be-included.xsl" the transformer does not
find it.

Also some trick like <xsl:include
href="concat($param,'to-be-included.xsl')"/> does not work, concat will
not be evaluated prior to parsing the URI - at least in my experiences.
($param contains the dynamically computed directory path.)

Also I did not find a way determine what a running transformer thinks
what actually is its "base URI".

Do you have an idea what basics I'm missing here?

Rolf