'Starting from each untranslated word, find the longest
continuous span of untranslated positions
:param sentence_length: Length of source sentence being
translated by the hypothesis
:type sentence_length: int
:rtype: list(tuple(int, int))'

def untranslated_spans(self, sentence_length):

    translated_positions = self.translated_positions()
    translated_positions.sort()
    translated_positions.append(sentence_length)
    untranslated_spans = []
    start = 0
    for end in translated_positions:
        if (start < end):
            untranslated_spans.append((start, end))
        start = (end + 1)
    return untranslated_spans
