def __init__(self, refs=set(), name=None, isFeature=False):
        """Initializes the set of known references, empty by default."""
        self.refs = refs
        self.featureNames = set()
        self.lookupNames = set()
        self.tableNames = set()
        self.languageSystems = set()
        super(FilterFeatureWriter, self).__init__(
            name=name, isFeature=isFeature)

        # error to print when undefined reference is found in glyph class
        self.classErr = ('Undefined reference "%s" removed from glyph class '
            'definition %s.')

        # error to print when undefined reference is found in sub or pos rule
        subErr = ['Substitution rule with undefined reference "%s" removed']
        if self._name:
            subErr.append(" from ")
            subErr.append("feature" if self._isFeature else "lookup")
            subErr.append(self._name % ' "%s"')
        subErr.append(".")
        self.subErr = "".join(subErr)
        self.posErr = self.subErr.replace("Substitution", "Positioning")
dataset/ETHPy150Open google/roboto/scripts/lib/fontbuild/features.py/FilterFeatureWriter.__init__/swapped operands of `%`