select element based on value of one of the children

XSL processor, processor version, and version of the language: I use IE6
to test my stylesheet and have the following header in my stylesheet
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

Dear reader,
As a XML and XSL newbie I would like to know whether it is possible to
select an element based on the value of one of children and how this can
best be done.

I have a xml document with the following structure:
<main>
	<a>
		<b>5</b>
		<c>G</c>
	</a>
	<a>
		<b>7</b>
		<c>L</c>
	</a>
	<a>
		<b>4</b>
		<c>T</c>
	</a>
</main>

I would like to know how to, for example, select the following element
and use it in my stylesheet (<xsl:for-each select="???????">):
<a>
	<b>7</b>
	<c>L</c>
</a>

Note that I select the element with a child b having a value of 7.

Cheers,

Bjorn

P.S. I tried to do my "homework" and found the following example
http://www.dpawson.co.uk/xsl/sect2/N3235.html#d4086e214. I, however, do
not understand what their solution to the problem is.