input: Write a python function to generate all permutations of a given word.
  assert set(permute_word("abc")) == set(["abc", "acb", "bac", "bca", "cab", "cba"])
  assert set(permute_word("a")) == set(["a"])
  assert set(permute_word("")) == set([""])
output: def permute_word(word):
      if len(word) <= 1:
          return [word]
      permutations = []
      for i, char in enumerate(word):
          for perm in permute_word(word[:i] + word[i+1:]):
              permutations.append(char + perm)
      return permutations