I'm having trouble writing a particular expression in R that uses complex subscripts. The expression should look like this: $(A_{1,2,L}, A_{1,2,U})$

I have an expression that produces the correct subscripts (example below) but I'm unable to add the comma between the entries. Do I need to use substitute() or group()?

plot(NULL, xlim = c(0, 2), ylim = c(0,2))
text(x = 1, y = 1, label = expression((A["1,2,L"]~A["1,2,U"])))