Stylesheet level variables - SourceTree is not accessible

["Roytman, Alex" <ro...@peacetech.com>]

Hello,

I am having some problems with global variables referencing source tree
nodes. 

I define a global variable $council to store external XML from
http://localhost/myservlet?council-id=<value> URL

<xsl:variable name="council"
select="document(concat('http://localhost/myservlet?council-id=',
/xsum/@council-id)/councils/council[1]"/>

/xsum/@council-id in concat() returns empty result and of course the
document() call fails because of invalid URL

(all works fine when I define this variable inside of <xsl:template>)

I noticed that the URL gets loaded twice - once for variable definition
(fails) and second time when I use $council variable (succeeds)

Am I right that Source tree is not available on stylesheet level and global
variables are calculated later when referenced first time?
If NOT, why /xsum/@council-id returns blank when used for stylesheet level
variables?
If YES, why value for $council variable gets calculated on variable
definition?

Or may be I missed something very simple?

Thank you in advance

Alex