Use 'count' or xsl:variable?

Hello,

I have an xml document of books like this...

<library>
   <book>
     <first_name>....</first_name>
     <last_name>.....</last_name>
     <title>.....</title>
     <ibn>.....</ibn>
     <cost>....</cost>
   </book>

etc etc
</library>

and need to count how many titles there are for every
instance of last_names. Is it better to initialize a
variable and increment it every time or just use the
'count' part of select?

Thank you

Marina

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