4.35 How to find parts of a Sum?

Given an inert Sum such as

\[ r = \sum _{n=2}^{\infty } a_{n -1} x^{n -1} \]

How to obtain the body of the sum, the index variable, the lower starting value and the upper limit?

r:=Sum(a[n - 1]*x^(n - 1), n = 2 .. infinity); 
op(0,r)  #head 
                              Sum 
op(1,r) #body 
                       a[n - 1] x^(n - 1) 
 
op(2,r) # sum specs 
                       n = 2 .. infinity 
 
lhs(op(2,r))  #name of summation index 
                               n 
 
rhs(op(2,r))  # lower..upper limits 
                         2 .. infinity 
 
op(1,rhs(op(2,r)))  #lower limit 
                               2 
 
op(2,rhs(op(2,r)))  #upper limit 
 
                    infinity