input: Write a function to count and return words from a given list that appear at least n times. If the list is empty or no word appears n times, return an empty list. The function should use a Counter from the collections module to speed up the counting process.
  assert frequent_words(['a', 'b', 'a', 'c', 'd', 'a', 'x', 'd', 'd'], 3)==['a', 'd']
  assert frequent_words(['a', 'a', 'a', 'b', 'b', 'b', 'c'], 2)==['a', 'b']
  assert frequent_words(['x', 'y', 'z'], 2)==[]
  assert frequent_words([], 1)==[]
  assert frequent_words(['a', 'b', 'c', 'd', 'e', 'f'], 1)==['a', 'b', 'c', 'd', 'e', 'f']
output: from collections import Counter
  
  def frequent_words(words, n):
      if not words:
          return []
      word_counts = Counter(words)
      return [word for word, count in word_counts.items() if count >= n]