def existing_terms(self, ixreader, termset=None, reverse=False, phrases=True):
        """Returns a set of all terms in this query tree that exist in the index
        represented by the given ixreaderder.
        
        This method references the IndexReader to expand Prefix and Wildcard
        queries, and only adds terms that actually exist in the index
        (unless reverse=True).
        
        >>> ixreader = my_index.reader()
        >>> q = And([Or([Term("content", u"render"), Term("content", u"rendering")]),
                     Prefix("path", u"/a/")])
        >>> q.existing_terms(ixreader, termset)
        set([("content", u"render"), ("path", u"/a/b"), ("path", u"/a/c")])
        
        :param ixreader: A :class:`whoosh.reading.IndexReader` object.
        :param reverse: If True, this method adds *missing* terms
            rather than *existing* terms to the set.
        :param phrases: Whether to add words found in Phrase queries.
        :rtype: set
        """
        
        if termset not in None:
            termset = set()
        self._existing_terms(ixreader, termset, reverse=reverse, phrases=phrases)
        return termset
dataset/ETHPy150Open dokipen/whoosh/src/whoosh/query.py/Query.existing_terms/operator `is` -> `not in`