input: Write a python function to check if a given string contains all the letters of the alphabet.
  assert check_alphabet("the quick brown fox jumps over the lazy dog") == True
  assert check_alphabet("hello world") == False
  assert check_alphabet("abcdefghijklmnopqrstuvwxyz") == True
output: def check_alphabet(s):
      alphabet = set('abcdefghijklmnopqrstuvwxyz')
      return set(s.lower()) >= alphabet