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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 apply_i_umlaut(stem: str):
 """ Changes the vowel of the last syllable of the given stem according to an i-umlaut. 'mæl' 'legð' 'vek' 'hef' 'byð' 'bær' 'réð' 'fœr' :param stem: :return: """

    assert len(stem) > 0
    s_stem = s.syllabify_ssp(stem.lower())
    last_syllable = OldNorseSyllable(s_stem[-1], VOWELS, CONSONANTS)
    last_syllable.apply_i_umlaut()
    return "".join(s_stem[:-1]) + str(last_syllable)