How to set Heading artifact numbering to start with alphabetic letters instead of numbers?

In a module in RDNG, how do you set the auto-numbering of Heading artifacts to be alphabetic letters instead of numbers. For example, after the artifacts in Section 8, I want an artifact of type “Heading” to be A.1 followed by artifacts within section A.1 to be A.1.1, A.1.2 etc. Similarly, Appendix B will have a heading of B.1, followed by artifacts within the section numbered B.1.1, B.1.2, etc.
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