>>>Since I write this value to a log file, I want to be sure that it is always a string, not a numeric value. Can you think of a case where program(program(-1)-1) will give a numeric value?
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>Whenever potentially different vartypes are involved you might consider a Transform() wrapper. Transform() has been included for years in West Wind script compilation where it avoids vartype errors at very low cost.

Transform is not reliable if it comes to terms of decimal points, decimal places, that is SET POINT, SET DECIMAL, SET FIXED. PAD[LR] works better if you like to gather a numbers real value.