def download_if_missing(self, version=None, verbose=True):
        """Download the jar for version into the jar_filename specified
        in the constructor. Will not overwrite jar_filename if it already
        exists. version defaults to DEFAULT_CORENLP_VERSION (ideally the
        latest but we can't guarantee that since PyStanfordDependencies
        is distributed separately)."""
        if os.path.exists(self.jar_filename):
            return

        jar_url = self.get_jar_url(version)
        if verbose:
            print("Downloading %r -> %r" % (jar_url, self.jar_filename))
        opener = ErrorAwareURLOpener()
        opener.retrieve(jar_url, filename=self.jar_filename)
Download the jar for version into the jar_filename specified
        in the constructor. Will not overwrite jar_filename if it already
        exists. version defaults to DEFAULT_CORENLP_VERSION (ideally the
        latest but we can't guarantee that since PyStanfordDependencies
        is distributed separately).