input: Write a python function to check if a given string contains every letter of the alphabet at least once.
  assert check_alphabet("The quick brown fox jumps over the lazy dog") == True
  assert check_alphabet("Hello World") == False
  assert check_alphabet("Pack my box with five dozen liquor jugs") == True
output: def check_alphabet(s):
      alphabet = set('abcdefghijklmnopqrstuvwxyz')
      return alphabet <= set(s.lower())