code: def next_key(tuple_of_tuples, key):
      
      for i, t in enumerate(tuple_of_tuples):
          if t[0] == key:
              try:
                  return tuple_of_tuples[i + 1][0]
              except IndexError:
                  return None
docs: Returns the key which comes after the given key.
  
      It processes a tuple of 2-element tuples and returns the key which comes
      after the given key.