I have a list $A = (1,2,3,4)$. The second entry can be denoted by $A_2=2$. Let $R$ be an operator that reverses a list, ie., $R[A] =(4,3,2,1)$. What is the notation for the second entry in $R[A]$? I don't think $R[A]_2$ is clear because it suggests that the operator has two entries. What do you suggest? I am currently writing $(R[A])_2$, but this looks like $((4,3,2,1))_2$, which doesn't make sense.
ps. I realize that to reverse a vector I could use the anti-diagonal matrix $J$ and then the second entry would be $J_{2,i} \cdot A_i$. Hence, I could avoid the need for an operator $R$. However, I'm only picking the reversal operator as a simple example. The operator can be more general.