How to pass xsl variable to external function?

[souri datta]

Hi,
   I am using xalan external function. I can pass normal strings to the
function like external:f('string1','string2')
But,if I want to pass the value of some previously defined xsl varible,how
to pass it?
I am not very familiar with xsl files.So,can anyone guide me in this?
Thanking in advance.