input: Write a function to return a list of words that appear at least n times in a given list of words. If the input list is empty then return an empty list.
  from collections import Counter
  
  def frequent_words(words, n):
      if not words:
          return []
      counts = Counter(words)
      return [word for word, count in counts.items() if count >= n]
  
  assert frequent_words(["a", "b", "a", "c", "d", "a", "x", "d", "d"], 3) == ["a", "d"]
  assert frequent_words(["a", "b", "a", "c"], 2) == ["a"]
  assert frequent_words(["x", "y", "z"], 1) == ["x", "y", "z"]
  assert frequent_words([], 2) == []
  assert frequent_words(["w", "w", "w", "w"], 4) == ["w"]
output: 
  from collections import Counter
  
  def frequent_words(words, n):
      if not words:
          return []
      counts = Counter(words)
      return [word for word, count in counts.items() if count >= n]
  