URI collection select?

I once upon a time found a documentation with examples on various way
to build collections and for the life of me I cannot find it again.

I have a situation where I have files with a .dita and .xml file
extensions. I thought there was a way to do this in one select
statement but all the ways I can think of do not work. I've tried
separating the extensions with a comma, a space and a colon and none
of them work - no results are returned.B 

So I have this configuration:

	B  B 

<xsl:variable
 name
=
"topiccollectionString"

B  B  B  B 
select
=
"_concat_
(
$SRCPATH
,

'?select=*.dita;recurse=no'
)
"/>

B  B 

<xsl:variable
 name
=
"topicfileSet" select
=
"_uri-collection_
(
$TOPICCOLLECTIONSTRING
)
"/>

 B  B 

<xsl:variable
 name
=
"xmltopiccollectionString"

B  B  B  B 
select
=
"_concat_
(
$SRCPATH
,

'?select=*.xml;recurse=no'
)
"/>

B  B 

<xsl:variable
 name
=
"xmltopicfileSet" select
=
"_uri-collection_
(
$XMLTOPICCOLLECTIONSTRING
)
"/>

	I then have 2 for-each statements to process both lists.