returns a list of all token IDs occuring the the given exmaralda file,
        sorted by their time stamp in ascending order.

def get_token_ids(tree):
        """
        returns a list of all token IDs occuring the the given exmaralda file,
        sorted by their time stamp in ascending order.
        """
        def tok2time(token_element):
            '''
            extracts the time (float) of a <tli> element
            (i.e. the absolute position of a token in the document)
            '''
            return float(token_element.attrib['time'])

        timeline = tree.find('//common-timeline')
        return (tok.attrib['id']
                for tok in sorted((tli for tli in timeline.iterchildren()),
                                  key=tok2time))