"sum" of "substring"

Hello

My problem:
I have a flat text file that has to be transformed into a predetermined XML
file.

Example:
TYPE1Field1AnotherField12.00YetAnotherField445.26FinalField
TYPE1ABCDEFABCDEFGHIJKL45.00ABCDEFGHIJKLMNO123.45ABCDEFGHIJ
TYPE2FieldAnother987.65Final

where the first 5 characters indicate which one of two record layouts it is.

I first do some rudimentary wrapping of the records, creating an
intermediary XML file that
I can then use XSLT to process:

<records>
<record>TYPE1Field1AnotherField12.0YetAnotherField445.26FinalField</record>
<record>TYPE1ABCDEFABCDEFGHIJKL45.0ABCDEFGHIJKLMNO123.45ABCDEFGHIJ</record>
<record>TYPE2FieldAnother987.65Final</record>
</records>


I can populate nodes in the final output XML file by extracting data via
"substring", such as 
<xsl:value-of select="substring(.,28,15)">

I need to total up certain numeric values within the record of a specific
layouts.
For example, I want to total up the values that occupy positions 24 thru 28
from all records of "TYPE1".
Is there any way of doing this?
I thought I should use "sum" but I can't figure out the syntax needed.

Kind regards
Colin Simpson

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