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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 surface(self, tag):
        ''' Get surface string that is associated with a tag object '''
        if tag.cfrom is not None and tag.cto is not None and tag.cfrom >= 0 and tag.cto >= 0:
            return self.text[tag.cfrom:tag.cto]
        else:
            return ''