Thx a lot Abel...
I have one last question
if i make something like this:

(xsl:for-each select="field[contains(@id, 'containerWidth')]")
 (xsl:variable name="var1" select="@value" /) 
(/xsl:for-each)

in var1 i put different values one for each instance of containerWidth in the xml tree, infact if i make:

(xsl:value-of select="$var1" /)

it print out this:
45
14
2
6

If i want to make one variable for each values or make a variable that contains only the first value how can i do?

in any case thx all guys this list is very usefull to me

bye 
Matteo