input: Write a function to find the words that appear at least 'n' times in a list of words.
  assert frequent_words(['a', 'b', 'a', 'c', 'd', 'd', 'a'], 2) == ['a', 'd']
  assert frequent_words(['apple', 'banana', 'apple', 'orange', 'banana', 'banana'], 3) == ['banana']
  assert frequent_words(['cat', 'dog', 'cat', 'cat', 'mouse', 'dog'], 2) == ['cat', 'dog']
output: def frequent_words(words, n):
      from collections import Counter
      word_count = Counter(words)
      return [word for word, count in word_count.items() if count >= n]