Help getting the difference of two attribute lists

Hi,
I have two lists of attributes ($attr1, $attr2). I want to make a list
($attr3) that will be everything found in $attr1 and NOT in $attr2.
example:

doc1.xml:

<attr1 a=5 b=6 c=7/>
<attr2 c=7 b=6/>


Running this code:

<xsl:variable name="attr1" select="attr1\@*"/>
<xsl:variable name="attr2" select="attr2\@*"/>


Returns this:

$attr1[0] = 'a' 5
$attr1[1] = 'b' 6
$attr1[2] = 'c' 7

$attr2[0] = 'c' 7 // Notice that its not ordered
$attr2[1] = 'b' 6

What I would like to get in attr3 is this:

$attr3[0] = 'a' 5 <- in attr1 and NOT in attr2

I looked around, and found some formula, that didnt work
$attr1[count(.|$attr2)=count($attr2)]..

<xsl:variable name="attr3"
select="$attr1[count(.|$attr2)=count($attr2)]"/>


attr3 came out empty..

If you have anything that might help me, please let me know, iam quite
lost..