[xsl] Java XSLT transformers and document('') problem

[LS <leonid@xxxxxxxxxxxxx>]

I have strange behavior with XSLT and Java (tested on Xalan and Saxon):

when I create transformer from XSLT file, function document('') works as 
I expect:

document('')/* -> xsl:template
count(document('')/*) -> 1

when I create transformer from Domsource, modify it (add some templates, 
remove others) - something breaks function document(''):

document('')/* -> ?
count(document('')/*) -> 0

all works well when I write modified XSL to file and create transformer 
from that file
but dumping modified XSL to string  and create transformer from 
Streamsourse again works with errors

Is there any way to create transformation on-the fly and use it without 
temporary dumping it to file?
My question depends only on document('') function, because all of the 
others seems like ok