Does anyone know how to cancel an Excel calculation sequence part-way
through, within a UDF?

I want to do the following:

1) Excel calculates a workbook that has several UDF calls that each
take a long time.

2) The user can cancel a call using a Cancel button on a pop-up window.
This I am doing OK already.

3) This should then cancel the sequence (as if they have pressed
Escape), so they don't have to press Cancel for each call that Excel
makes to each UDF.