code: def interval(key, start_note, interval):
      
      if not notes.is_valid_note(start_note):
          raise KeyError("The start note '%s' is not a valid note" % start_note)
      notes_in_key = keys.get_notes(key)
      for n in notes_in_key:
          if n[0] == start_note[0]:
              index = notes_in_key.index(n)
      return notes_in_key[(index + interval) % 7]
docs: Return the note found at the interval starting from start_note in the
      given key.
  
      Raise a KeyError exception if start_note is not a valid note.
  
      Example:
      >>> interval('C', 'D', 1)
      'E'