[xsl] (newby) define an attribute based on an attribute value

Hi,

I'm a newby in XSL tranformations. I need to trasform an XML file into
another XML file with a different structure.

Inside a template I build an element named Point that has several
attributes. The value of one of these attributes depends on the value
of an attribute in the source XML (the attribute v of the element B
that I filtered through the parent 'id', in the code below)

[snip]
   <xsl:element name="Point">
[snip]
      <xsl:attribute name="PointAttribute">
        <xsl:value-of select="A[@id='123']/B[@v]"/>
      </xsl:attribute>
[snip]
   </xsl:element>
[snip]

I would like to obtain a 'Point' element with an attribute called
'PointAttribute' with the same value of the 'v' attribute of the
element 'B'.

What is the best way to deal with this problem?

I thank you for the help.
Best regards.

Livio.