Write a function that takes, as an argument, a string, and returns a list of all of the unique letters in the string. If the string contains digits or punctuation, do not include those symbols in the list that is returned. Name this function getLetters(myString).

For example,

>>>getLetters(hello, world)

should return the list [h, e, l, o, w, r, d]. As another example,

>>>getLetters(123)should return the empty list, [].