input: Write a python function to find the indices of occurrences of the substring "cat" in a given sentence.
  assert find_occurrences("The cat sat on the mat.") == [4]
  assert find_occurrences("catcatcat") == [0, 3, 6]
  assert find_occurrences("concatenate") == [3]
  assert find_occurrences("The dog is in the yard.") == []
output: def find_occurrences(sentence):
      indices = []
      index = sentence.find("cat")
      while index != -1:
          indices.append(index)
          index = sentence.find("cat", index + 1)
      return indices