Repopulating a list/array

In my program, I have 4 blocks with 100 trials each. I have it set so that with each trial I draw an item from a list like below:

list = ["a","b","c"]; // this is in Begin Experiment
x = list.pop() // this is in Begin Routine

By the end of the first block the list runs out of items, so I’d like to repopulate the list at the beginning of each block. Does anyone have suggestions on how to do that?