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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 _get_evidence_text(self, event_tag):
 """Extract the evidence for an event. Pieces of text linked to an EVENT are fragments of a sentence. The EVENT refers to the paragraph ID and the "uttnum", which corresponds to a sentence ID. Here we find and return the full sentence from which the event was taken. """

        par_id = event_tag.attrib.get('paragraph')
        uttnum = event_tag.attrib.get('uttnum')
        event_text = event_tag.find('text')
        if self.sentences is not None and uttnum is not None:
            sentence = self.sentences[uttnum]
        elif event_text is not None:
            sentence = event_text.text
        else:
            sentence = None
        return sentence