Question: Converting strings of letters into a list of letters?

How would I generate all possible strings of letters from a given string of letters? (For example, if a given string of letters is "AA", then all possible strings of letters are "A" and "AA". If a given string of letters is "ABA", then all possible strings of letters in lexicographic order are "A", "AA", "AAB", "AB", "ABA", "B", "BA", "BAA".) I know that I first need to convert the string of letters to a list of letters, but I can't figure out the correct coding. Thanks for any advice!
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