XPath context evaluation

I apologize for hijacking the XSL list for XPath questions, I wish there
would be some kind of XPath list ..

I have written some XSL-like application (maybe a bit more like FOP)
which uses the results of XPath expression as trigger for further
processing:
The attached file is a stripped down example of an XML file I would like
to process. At some point I am iterating over the <some> element and
it's children.
I would like to retrieve the associated <info> element from the
<infoList> for a given <id> element in the <idList>.
Since the application is agnostic about the data it reads I want to use
an XPath expression to find the correct element. Using a string as '123'
(e.g. "//info[id = '123']" ) shows no issues, however I am not able to
use the current context (one of the <id> elements in the <idList>) to
evaluate an XPath expression as "//info[id = .]" or "//info[string(id) =
(.)]".

Kindest regards,
Victor

<?xml version="1.0" encoding="UTF-8"?>
<root>
	<infoList>
		<info>
			<name>abc</name>
			<id>123</id>
		</info>
		<info>
			<name>klm</name>
			<id>456</id>
		</info>
		<info>
			<name>xyz</name>
			<id>789</id>
		</info>
	</infoList>
	<data>
		<someList>
			<some>
				<readme>some words</readme>
				<idList>
					<id>123</id>
				</idList>
			</some>
			<some>
				<readme>some more words</readme>
				<idList>
					<id>123</id>
					<id>456</id>
				</idList>
			</some>
			<some>
				<readme>famous last words</readme>
				<idList>
					<id>456</id>
					<id>789</id>
				</idList>
			</some>
		</someList>
	</data>
</root>