def accentuate(letter):
    """ Function that adds the accentuation mark of a letter:

        :param letter: a letter to be accentuated
        :type letter: str
        :returns: letter accentuated
        :rtype: str

        :Example:

        >>> import chana.syllabificator
        >>> chana.syllabificator.accentuate('a')
        á

    """
    if letter == 'a':
        letter = 'á'
    elif letter == 'e':
        letter = 'é'
    elif letter == 'i':
        letter = 'í'
    elif letter == 'o':
        letter = 'ó'
    elif letter == 'u':
        letter = 'ú'
    return letter
