[NTG-context] a question about formula labels: letters preceding numbers

From the documentation, I can see how to produce labels such as "1S", with the "S" following the equation number, but I don't yet see how to make the "S" come in front. Since there are only two such equations in

I'd like to typeset equations like these: <some formula> (S1) <another formula> (S2) the particular document that I'm working with, a brutish one-off solution would suffice. But a method for prefixing a letter to a number (rather than the other way around) would be nicer. Thanks, Jesse -- Jesse Alama (alama@stanford.edu)