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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def interval(key, start_note, interval):
 """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: 'E' """

    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]