title: checking if string only contains certain letters in Python
body: <p>i'm trying to write a program that completes the MU game
  <a href="https://en.wikipedia.org/wiki/MU_puzzle" rel="nofollow">https://en.wikipedia.org/wiki/MU_puzzle</a></p>
  
  <p>basically i'm stuck with ensuring that the user input contains ONLY M, U and I characters.</p>
  
  <p>i've written</p>
  
  <pre><code>alphabet = ('abcdefghijklmnopqrstuvwxyz')
  
  string = input("Enter a combination of M, U and I: ")
  
  if "M" and "U" and "I" in string:
      print("This is correct")
  else:
      print("This is invalid")
  </code></pre>
  
  <p>i only just realised this doesnt work because its not exclusive to just M U and I. can anyone give me a hand?</p>