Maintaining large Excel workbooks is dull and thankless work. The situation can be can be somewhat improved with custom functions whose logic can be tested outside of Excel. Traditionally this has been a fairly advanced area with not a huge amount of adoption. xllify can create high performance .xll add-ins to extend Excel, all from prompting with requirements, existing formulas or VBA. The resulting add-in uses the C SDK for maximum performance: no Python, .NET or COM. I'd be glad of any thoughts!