Retrieve Indices of Associative Array as Collection

Question

In PL/SQL, suppose I have some associative array defined as follows:

declare 
       type a_arr_t is table of PLS_INTEGER index by PLS_INTEGER;
       a_arr a_arr_t;

I then, sparsely, populate the array as follows:

begin
      a_arr(1)   := 2;
      a_arr(10)  := 4;
      a_arr(100) := 6;
end;

Is there some operator or function that can give me the indices of the array, (1,10,100) as a collection of some sort, something like indices of in a forall statement?