code: def find_greeting(s):
      
      if s[0] == 'H':
          if s[:3] in ['Hi', 'Hi ', 'Hi,', 'Hi!']:
              return s[:2]
          elif s[:6] in ['Hello', 'Hello ', 'Hello,', 'Hello!']:
              return s[:5]
      elif s[0] == 'Y':
          if s[1] == 'o' and s[:3] in ['Yo', 'Yo,', 'Yo ', 'Yo!']:
              return s[:2]
      return None
docs: Return the the greeting string Hi, Hello, or Yo if it occurs at the beginning of a string
  
      >>> find_greeting('Hi Mr. Turing!')
      'Hi'
      >>> find_greeting('Hello, Rosa.')
      'Hello'
      >>> find_greeting("Yo, what's up?")
      'Yo'
      >>> find_greeting("Hello")
      'Hello'
      >>> print(find_greeting("hello"))
      None
      >>> print(find_greeting("HelloWorld"))
      None