input: Write a python function to classify an animal based on its features.
  assert classify_animal(["four legs", "curved tail", "feathers"]) == "bird"
  assert classify_animal(["four legs", "curved tail"]) == "mammal"
  assert classify_animal(["two legs", "feathers"]) == "unknown"
  assert classify_animal(["four legs"]) == "unknown"
output: def classify_animal(features):
      if "feathers" in features:
          if "four legs" in features and "curved tail" in features:
              return "bird"
          else:
              return "unknown"
      elif "four legs" in features and "curved tail" in features:
          return "mammal"
      else:
          return "unknown"