input: Write a python function to generate all permutations of a list of letters.
  assert permute_letters(['a', 'b']) == [['a', 'b'], ['b', 'a']]
  assert permute_letters(['a', 'b', 'c']) == [['a', 'b', 'c'], ['a', 'c', 'b'], ['b', 'a', 'c'], ['b', 'c', 'a'], ['c', 'a', 'b'], ['c', 'b', 'a']]
  assert permute_letters(['x']) == [['x']]
  assert permute_letters([]) == []
output: def permute_letters(letters):
      if len(letters) == 0:
          return [[]]
      if len(letters) == 1:
          return [letters]
      
      permutations = []
      for i in range(len(letters)):
          current_letter = letters[i]
          remaining_letters = letters[:i] + letters[i+1:]
          for p in permute_letters(remaining_letters):
              permutations.append([current_letter] + p)
      
      return permutations