[xsl] How to check if a value is numeric and add 2500 to it?

Assume I have a XML doc which might contain an optional tag with a numeric 
value similar to

<ns1:aaa>345</ns1:aaa>

Now I want to check with a XSLT expression:
- if this optional <ns1:aaa> tag exists at all

- if it exists then it should be checked if the value is numeric (leading and 
trainling blanks should be ignored. integer and floating numbers should both be 
acceptable

- If the value is numeric then 2500 (resp. 2500.00) should be added

How can I implement this logic in XSLT?

Thank you
Ben



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