I have found a module which starts as follows:-
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' This module contains routines for cubic spline interpolation and
integration.

' Designed for Microsoft Excel 97 and beyond

' Written 1999/6/30, David J. Braden

' Revisions:

' 1999/7/2 DJB Made 2nd derivative at upper endpoint exact

' Tightened up input validation code

' 1999/7/6 DJB Streamlined (optional) error-checking

' About 20% of the code is for checking that input is valid, hence the length.

' If you "know" that input is always valid, set the compiler directive

' fValidateInput = True.
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etc. etc

what exactly do I need to do to add this module which contains functions and
subroutines to an excel workbook 2003