':return: Zero-indexed alignment, suitable for use in external
``nltk.translate`` modules like ``nltk.translate.Alignment``
:rtype: list(tuple)'

def zero_indexed_alignment(self):

    zero_indexed_alignment = []
    for j in range(1, len(self.trg_sentence)):
        i = (self.alignment[j] - 1)
        if (i < 0):
            i = None
        zero_indexed_alignment.append(((j - 1), i))
    return zero_indexed_alignment
