[Silgraphite-fonts] How to refer to corresponding letter in separate class

[<ek...@di...>]

Suppose I had two classes

uppercase = codepoint("ABCDEFGHIJKLMNOPQRSTUVWXYZ");
lowercase = codepoint("abcdefghijklmnopqrstuvwxyz");

and I wanted to convert every uppercase letter to a "S" if it's preceded by the corresponding lowercase letter (so "bB" becomes "bS", "fF" becomes "fS", but "cD" stays as "cD"), how would I do that?

Doing `lowercase uppercase > @1 U+0053` doesn't work because then any combination of lowercase and uppercase becomes lowercase + S including "cD" becoming "cS" (and I only want the corresponding uppercase to match)

Doing `uppercase > U+0053 / lowercase _;` doesn't work either for the same reason.

I can't figure it out. Can anyone please help me out?